Rendering Screen problem?

What I expected to happen was...:
I went to single player and when I tried to load my game I got this error screen. I don't use any mods.

Minecraft has crashed!      
      ----------------------      

Minecraft has stopped running because it encountered a problem; Rendering screen

We were unable to save this report to a file.



--- BEGIN ERROR REPORT 62808a77 --------
Generated 2/27/13 7:58 PM

-- Head --
Stacktrace:
	at ahw.h(SourceFile:49)
	at ahw.<init>(SourceFile:35)
	at ahs.<init>(SourceFile:16)
	at aht.a(SourceFile:78)
	at net.minecraft.client.Minecraft.a(SourceFile:1371)
	at avb.e(SourceFile:157)
	at avc.a(SourceFile:205)
	at ava.a(SourceFile:175)
	at avb.a(SourceFile:178)

-- Screen render details --
Details:
	Screen name: bfm
	Mouse location: Scaled: (179, 102). Absolute: (359, 274)
	Screen size: Scaled: (427, 240). Absolute: (854, 480). Scale factor of 2
Stacktrace:
	at bbb.b(SourceFile:774)
	at net.minecraft.client.Minecraft.J(SourceFile:605)
	at net.minecraft.client.Minecraft.run(SourceFile:534)
	at java.lang.Thread.run(Unknown Source)

-- System Details --
Details:
	Minecraft Version: 13w01a
	Operating System: Windows 7 (amd64) version 6.1
	Java Version: 1.7.0_15, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 479485688 bytes (457 MB) / 514523136 bytes (490 MB) up to 954466304 bytes (910 MB)
	JVM Flags: 2 total; -Xms512m -Xmx1024m
	AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	LWJGL: 2.4.2
	OpenGL: AMD Radeon HD 6310 Graphics                      GL version 4.1.10834 Compatibility Profile Context, ATI Technologies Inc.
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Texture Pack: Sphax PureBDCraft 128x MC14
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: ~~ERROR~~ NullPointerException: null

java.lang.RuntimeException: Failed to check session lock, aborting
	at ahw.h(SourceFile:49)
	at ahw.<init>(SourceFile:35)
	at ahs.<init>(SourceFile:16)
	at aht.a(SourceFile:78)
	at net.minecraft.client.Minecraft.a(SourceFile:1371)
	at avb.e(SourceFile:157)
	at avc.a(SourceFile:205)
	at ava.a(SourceFile:175)
	at avb.a(SourceFile:178)
	at bbb.b(SourceFile:774)
	at net.minecraft.client.Minecraft.J(SourceFile:605)
	at net.minecraft.client.Minecraft.run(SourceFile:534)
	at java.lang.Thread.run(Unknown Source)
--- END ERROR REPORT 6f41caa9 ----------