Failed to check session lock, aborting

Fix:
Delete the file [minecraft|http://hopper.minecraft.net/help/finding-minecraft-data-folder]/saves/<worldname>/session.lock

If you need more support, please use the Mojang Support Center.

this is the crash report...

---- Minecraft Crash Report ----
// Would you like a cupcake?

Time: 22/02/13 4:23 PM
Description: Updating screen events

java.lang.RuntimeException: Failed to check session lock, aborting
	at ahv.h(SourceFile:49)
	at ahv.<init>(SourceFile:35)
	at ahr.<init>(SourceFile:16)
	at ahs.a(SourceFile:78)
	at net.minecraft.client.Minecraft.a(SourceFile:1370)
	at auo.e(SourceFile:157)
	at auo.a(SourceFile:122)
	at aul.a(SourceFile:72)
	at aul.d(SourceFile:127)
	at aul.m(SourceFile:111)
	at net.minecraft.client.Minecraft.l(SourceFile:1086)
	at net.minecraft.client.Minecraft.J(SourceFile:582)
	at net.minecraft.client.Minecraft.run(SourceFile:534)
	at java.lang.Thread.run(Unknown Source)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
	at ahv.h(SourceFile:49)
	at ahv.<init>(SourceFile:35)
	at ahr.<init>(SourceFile:16)
	at ahs.a(SourceFile:78)
	at net.minecraft.client.Minecraft.a(SourceFile:1370)
	at auo.e(SourceFile:157)
	at auo.a(SourceFile:122)
	at aul.a(SourceFile:72)
	at aul.d(SourceFile:127)
	at aul.m(SourceFile:111)

-- Affected screen --
Details:
	Screen name: bey
Stacktrace:
	at net.minecraft.client.Minecraft.l(SourceFile:1086)
	at net.minecraft.client.Minecraft.J(SourceFile:582)
	at net.minecraft.client.Minecraft.run(SourceFile:534)
	at java.lang.Thread.run(Unknown Source)

-- System Details --
Details:
	Minecraft Version: 1.4.7
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.6.0_30, Sun Microsystems Inc.
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Sun Microsystems Inc.
	Memory: 486058432 bytes (463 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 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 7640G GL version 4.2.11476 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: Default
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: ~~ERROR~~ NullPointerExce