My worlds are gone from my worlds list

Incomplete
Missing the crash reports

How do I find them? Are they saved somewhere on my pc?

%appdata% - Roaming - .minecraft - saves

Where do I find the crash reports?

in the /crash-reports/ folder inside your .minecraft folder.

Here's the latest one.

Minecraft Crash Report ----
// You're mean.

Time: 10-07-14 16:00
Description: Exception ticking world

java.lang.IllegalArgumentException: Cannot get property bcl{name=age, clazz=class java.lang.Integer, values=[0, 1, 2, 3, 4, 5, 6, 7]} as it does not exist!
at bbw.b(SourceFile:91)
at ayt.b(SourceFile:127)
at ayt.c(SourceFile:69)
at ary.b(SourceFile:416)
at qg.h(SourceFile:373)
at qg.c(SourceFile:199)
at net.minecraft.server.MinecraftServer.x(SourceFile:578)
at net.minecraft.server.MinecraftServer.w(SourceFile:510)
at cwg.w(SourceFile:132)
at net.minecraft.server.MinecraftServer.run(SourceFile:426)
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 bbw.b(SourceFile:91)
at ayt.b(SourceFile:127)
at ayt.c(SourceFile:69)
at ary.b(SourceFile:416)
at qg.h(SourceFile:373)
at qg.c(SourceFile:199)

– Affected level –
Details:
Level name: Testverden
All players: 1 total; [qj['Iben_dk'/373, l='Testverden', x=-130,88, y=68,00, z=213,18]]
Chunk stats: ServerChunkCache: 625 Drop: 0
Level seed: 4180892511208102622
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: -120,00,64,00,252,00 - World: (-120,64,252), Chunk: (at 8,4,12 in -8,15; contains blocks -128,0,240 to -113,255,255), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Level time: 766236 game time, 936667 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 174668 (now: false), thunder time: 117385 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
at net.minecraft.server.MinecraftServer.x(SourceFile:578)
at net.minecraft.server.MinecraftServer.w(SourceFile:510)
at cwg.w(SourceFile:132)
at net.minecraft.server.MinecraftServer.run(SourceFile:426)
at java.lang.Thread.run(Unknown Source)

– System Details –
Details:
Minecraft Version: 14w28a
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_60, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
Memory: 131949192 bytes (125 MB) / 276267008 bytes (263 MB) up to 518979584 bytes (494 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [qj['Iben_dk'/373, l='Testverden', x=-130,88, y=68,00, z=213,18]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

And the one right before that one:

Minecraft Crash Report ----
// You should try our sister game, Minceraft!

Time: 10-07-14 15:57
Description: Exception ticking world

java.lang.IllegalArgumentException: Cannot get property bcl{name=age, clazz=class java.lang.Integer, values=[0, 1, 2, 3, 4, 5, 6, 7]} as it does not exist!
at bbw.b(SourceFile:91)
at ayt.b(SourceFile:127)
at ayt.c(SourceFile:69)
at ary.b(SourceFile:416)
at qg.h(SourceFile:373)
at qg.c(SourceFile:199)
at net.minecraft.server.MinecraftServer.x(SourceFile:578)
at net.minecraft.server.MinecraftServer.w(SourceFile:510)
at cwg.w(SourceFile:132)
at net.minecraft.server.MinecraftServer.run(SourceFile:426)
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 bbw.b(SourceFile:91)
at ayt.b(SourceFile:127)
at ayt.c(SourceFile:69)
at ary.b(SourceFile:416)
at qg.h(SourceFile:373)
at qg.c(SourceFile:199)

– Affected level –
Details:
Level name: Testverden
All players: 1 total; [qj['Iben_dk'/375, l='Testverden', x=-130,88, y=68,00, z=213,18]]
Chunk stats: ServerChunkCache: 625 Drop: 0
Level seed: 4180892511208102622
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: -120,00,64,00,252,00 - World: (-120,64,252), Chunk: (at 8,4,12 in -8,15; contains blocks -128,0,240 to -113,255,255), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Level time: 766054 game time, 936485 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 174850 (now: false), thunder time: 117567 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
at net.minecraft.server.MinecraftServer.x(SourceFile:578)
at net.minecraft.server.MinecraftServer.w(SourceFile:510)
at cwg.w(SourceFile:132)
at net.minecraft.server.MinecraftServer.run(SourceFile:426)
at java.lang.Thread.run(Unknown Source)

– System Details –
Details:
Minecraft Version: 14w28a
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_60, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
Memory: 122519016 bytes (116 MB) / 282206208 bytes (269 MB) up to 518979584 bytes (494 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [qj['Iben_dk'/375, l='Testverden', x=-130,88, y=68,00, z=213,18]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

And your worlds are not there anymore?
Create a New World with the exact Name than one of the vanished one. That should recreate the world. Other than that, it's probably not fixable, and can be seen as one of the dangers of using Snapshots.

Wow! That worked! Thanks 🙂