Hostile Mobs won't spawn after a few minecraft days. I have hear that this only happens to lower end computers ( such as mine) but I have not found a solution. (The spawning also doesnt work with monster spawners).

Please type

/gamerule doMobSpawning true

and make sure the difficulty is not on Peaceful. If that doesn't work, please force a crash by holding F3+C for 10 or more seconds and attach the report here.

Also see MC-42053/MC-2536.

-- Head --
Stacktrace:
	at cem.a(SourceFile:308)

-- Affected level --
Details:
	Level name: MpServer
	All players: 1 total; [cin['Witonisaurus'/422, l='MpServer', x=121.52, y=70.00, z=232.79]]
	Chunk stats: MultiplayerChunkCache: 35, 35
	Level seed: 0
	Level generator: ID 04 - customized, ver 0. Features enabled: false
	Level generator options: 
	Level spawn location: 47.00,64.00,318.00 - World: (47,64,318), Chunk: (at 15,4,14 in 2,19; contains blocks 32,0,304 to 47,255,319), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
	Level time: 410697 game time, 457087 day time
	Level dimension: 0
	Level storage version: 0x00000 - Unknown?
	Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
	Forced entities: 60 total; [afv['Skeleton'/948, l='MpServer', x=109.50, y=24.00, z=283.50], agh['Zombie'/949, l='MpServer', x=112.50, y=24.00, z=282.50], adx['item.tile.dirt.default'/287, l='MpServer', x=139.97, y=68.00, z=233.34], adx['item.tile.dirt.default'/286, l='MpServer', x=136.88, y=69.00, z=232.53], adx['item.tile.dirt.default'/285, l='MpServer', x=129.72, y=67.00, z=226.38], acm['Sheep'/284, l='MpServer', x=146.06, y=69.00, z=225.13], adl['entity.ItemFrame.name'/258, l='MpServer', x=113.50, y=59.50, z=231.03], adl['entity.ItemFrame.name'/259, l='MpServer', x=112.50, y=59.50, z=240.97], agh['Zombie'/263, l='MpServer', x=111.06, y=24.00, z=282.84], afv['Skeleton'/260, l='MpServer', x=116.50, y=17.00, z=277.50], agh['Zombie'/261, l='MpServer', x=118.09, y=23.00, z=280.44], acd['Rabbit'/304, l='MpServer', x=152.78, y=71.00, z=199.44], agh['Zombie'/654, l='MpServer', x=109.91, y=24.00, z=281.56], abv['Cow'/306, l='MpServer', x=157.50, y=77.00, z=305.50], afv['Skeleton'/651, l='MpServer', x=106.50, y=24.00, z=283.50], acd['Rabbit'/311, l='MpServer', x=175.50, y=70.00, z=215.91], acd['Rabbit'/310, l='MpServer', x=170.63, y=72.00, z=206.25], acd['Rabbit'/313, l='MpServer', x=174.41, y=70.00, z=247.88], acd['Rabbit'/312, l='MpServer', x=174.75, y=66.00, z=245.69], acd['Rabbit'/315, l='MpServer', x=164.06, y=66.00, z=262.34], acd['Rabbit'/314, l='MpServer', x=172.88, y=65.00, z=251.56], acd['Rabbit'/317, l='MpServer', x=171.78, y=72.00, z=268.22], acd['Rabbit'/316, l='MpServer', x=166.25, y=71.00, z=265.09], abv['Cow'/319, l='MpServer', x=165.94, y=71.00, z=302.03], abv['Cow'/318, l='MpServer', x=167.19, y=70.00, z=299.84], adx['item.tile.dirt.default'/288, l='MpServer', x=131.53, y=69.00, z=225.50], adx['item.tile.dirt.default'/289, l='MpServer', x=138.28, y=70.00, z=235.13], adx['item.tile.dirt.default'/290, l='MpServer', x=130.03, y=67.00, z=224.88], adx['item.tile.dirt.default'/291, l='MpServer', x=134.63, y=70.00, z=234.56], adx['item.tile.dirt.default'/292, l='MpServer', x=128.31, y=68.00, z=224.44], adx['item.tile.dirt.default'/293, l='MpServer', x=128.22, y=66.00, z=228.47], adx['item.tile.dirt.default'/294, l='MpServer', x=138.09, y=69.00, z=231.16], aeq['Creeper'/296, l='MpServer', x=143.97, y=20.00, z=301.38], cin['Witonisaurus'/422, l='MpServer', x=121.52, y=70.00, z=232.79], abv['Cow'/223, l='MpServer', x=98.69, y=68.00, z=194.72], acd['Rabbit'/320, l='MpServer', x=175.13, y=69.00, z=297.78], aeq['Creeper'/754, l='MpServer', x=172.50, y=22.00, z=192.50], aeq['Creeper'/752, l='MpServer', x=170.50, y=22.00, z=193.50], aeq['Creeper'/753, l='MpServer', x=171.50, y=22.00, z=194.50], acd['Rabbit'/208, l='MpServer', x=94.09, y=66.00, z=267.50], abv['Cow'/239, l='MpServer', x=107.06, y=64.00, z=236.69], acm['Sheep'/238, l='MpServer', x=107.59, y=64.00, z=234.19], abv['Cow'/237, l='MpServer', x=105.34, y=64.00, z=233.66], abv['Cow'/236, l='MpServer', x=108.38, y=64.00, z=237.75], abv['Cow'/235, l='MpServer', x=109.53, y=64.00, z=237.05], abv['Cow'/234, l='MpServer', x=108.35, y=64.00, z=235.88], abv['Cow'/233, l='MpServer', x=106.03, y=64.00, z=234.91], abv['Cow'/232, l='MpServer', x=104.06, y=64.00, z=233.44], abv['Cow'/231, l='MpServer', x=104.06, y=64.00, z=234.91], abv['Cow'/230, l='MpServer', x=107.06, y=64.00, z=237.91], adl['entity.ItemFrame.name'/229, l='MpServer', x=109.03, y=59.50, z=232.50], adl['entity.ItemFrame.name'/228, l='MpServer', x=108.03, y=59.50, z=234.50], adl['entity.ItemFrame.name'/227, l='MpServer', x=107.03, y=59.50, z=236.50], adl['entity.ItemFrame.name'/226, l='MpServer', x=111.50, y=59.50, z=231.03], adl['entity.ItemFrame.name'/225, l='MpServer', x=107.03, y=58.50, z=236.50], adl['entity.ItemFrame.name'/224, l='MpServer', x=108.03, y=59.50, z=238.50], aeq['Creeper'/244, l='MpServer', x=104.03, y=24.00, z=286.50], adl['entity.ItemFrame.name'/242, l='MpServer', x=110.50, y=59.50, z=240.97], abv['Cow'/240, l='MpServer', x=107.13, y=64.00, z=235.50], abv['Cow'/241, l='MpServer', x=109.78, y=64.00, z=238.03]]
	Retry entities: 0 total; []
	Server brand: vanilla
	Server type: Integrated singleplayer server
Stacktrace:
	at cem.a(SourceFile:308)
	at bss.b(SourceFile:2278)
	at bss.a(SourceFile:326)
	at net.minecraft.client.main.Main.main(SourceFile:124)

-- System Details --
Details:
	Minecraft Version: 1.8.1-pre4
	Operating System: Windows XP (x86) version 5.1
	Java Version: 1.7.0_09, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
	Memory: 221198520 bytes (210 MB) / 380792832 bytes (363 MB) up to 523501568 bytes (499 MB)
	JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
	IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
	Launched Version: 1.8.1-pre4
	LWJGL: 2.9.1
	OpenGL: Intel(R) G45/G43 Express Chipset GL version 2.1.0 - Build 8.15.10.2555, Intel
	GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because EXT_framebuffer_object is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported.

	Using VBOs: No
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Resource Packs: [Blocksmith+Hybrid+v1.6c]
	Current Language: Canadian English (Canada)
	Profiler Position: N/A (disabled)

I have hear that this only happens to lower end computers

That's nonsense.

If friendly mobs spawn but not hostile then the difficulty is set (maybe locked) to peaceful.

Unrelated to that:

Your Java version is out of date! Please update your Java to the latest version (1.7.0_72 / 1.8.0_25): http://www.java.com/en/download/.

• When using a 32-bit browser on a 64-bit operating system the automatic version detection will only show the 32-bit version of Java, the 64-bit version needs to be downloaded manually: http://www.java.com/en/download/manual.jsp.

It definitely is not on peaceful, I've tried with the command and with the settings, it is not on peaceful.

On an unrelated note, I guess I'll update my Java soon!

Then it's a duplicate of MC-42053