Game crashes upon opening of previously saved world

Minecraft Crash Report ----
// I just don't know what went wrong 😞

Time: 7/31/13 5:43 PM
Description: Updating screen events

java.lang.RuntimeException: Failed to check session lock, aborting
at aln.h(SourceFile:44)
at aln.<init>(SourceFile:30)
at alj.<init>(SourceFile:16)
at alk.a(SourceFile:83)
at ats.a(SourceFile:1417)
at awe.e(SourceFile:157)
at awe.a(SourceFile:122)
at awb.a(SourceFile:69)
at awb.d(SourceFile:128)
at awb.m(SourceFile:107)
at ats.k(SourceFile:1158)
at ats.S(SourceFile:663)
at ats.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 aln.h(SourceFile:44)
at aln.<init>(SourceFile:30)
at alj.<init>(SourceFile:16)
at alk.a(SourceFile:83)
at ats.a(SourceFile:1417)
at awe.e(SourceFile:157)
at awe.a(SourceFile:122)
at awb.a(SourceFile:69)
at awb.d(SourceFile:128)
at awb.m(SourceFile:107)

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

– System Details –
Details:
Minecraft Version: 1.6.2
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
Memory: 64154480 bytes (61 MB) / 93822976 bytes (89 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: 1.6.2
LWJGL: 2.9.0
OpenGL: GeForce GTX 660/PCIe/SSE2 GL version 4.3.0, NVIDIA Corporation
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

Put the summary of the bug you're having here

What I expected to happen was...:
The game starting normally as it has for the last week or so.

What actually happened was...:
Game crashed upon clicking my saved world, and gave me the crash report page.