MINECRAFT CRASH

Minecraft Crash Report ----
// Oh - I know what I did wrong!

Time: 5/10/13 7:39 AM
Description: Updating screen events

java.lang.RuntimeException: Failed to check session lock, aborting
at alq.h(SourceFile:44)
at alq.<init>(SourceFile:30)
at alm.<init>(SourceFile:16)
at aln.a(SourceFile:83)
at atv.a(SourceFile:1417)
at avb.a(SourceFile:202)
at avb.a(SourceFile:293)
at awe.n(SourceFile:149)
at awe.m(SourceFile:111)
at atv.k(SourceFile:1158)
at atv.S(SourceFile:663)
at atv.d(SourceFile:619)
at net.minecraft.client.main.Main.main(SourceFile:101)

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

– Head –
Stacktrace:
at alq.h(SourceFile:44)
at alq.<init>(SourceFile:30)
at alm.<init>(SourceFile:16)
at aln.a(SourceFile:83)
at atv.a(SourceFile:1417)
at avb.a(SourceFile:202)
at avb.a(SourceFile:293)
at awe.n(SourceFile:149)
at awe.m(SourceFile:111)

– Affected screen –
Details:
Screen name: blt
Stacktrace:
at atv.k(SourceFile:1158)
at atv.S(SourceFile:663)
at atv.d(SourceFile:619)
at net.minecraft.client.main.Main.main(SourceFile:101)

– System Details –
Details:
Minecraft Version: 1.6.4
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_40, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
Memory: 102108016 bytes (97 MB) / 131612672 bytes (125 MB) up to 518979584 bytes (494 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
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
Launched Version: 1.6.4
LWJGL: 2.9.0
OpenGL: AMD Radeon HD 7670M GL version 4.2.11411 Compatibility Profile Context, ATI Technologies Inc.
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Pack: Default
Current Language: English (US)
Profiler Position: N/A (disabled)
Vec3 Pool Size: ~_ERROR~ NullPointerException: null