I had the same problem on a Windows server. My friends and I did not want to lose our world. I fixed it by downloading the latest Minecraft_server.exe. I renamed my original server folder from "Server" to "ServerOld" then created a New folder called "Server". Moved the new Minecraft_Server.exe to the new Server folder. From the ServerOld folder I copied the "world" folder into the new Server folder. Once I executed Minecraft_Server.exe all was back to normal again.
Hope this helps.
Had a similar error on a server designed for testing, I used MC edit to copy the valued creations to a new world.
Good luck with your worlds!
Error:
2011-10-24 19:01:28 [SEVERE] Unexpected exception
java.lang.NullPointerException
at en.c(SourceFile:72)
at eh.a(SourceFile:2124)
at jf.d(SourceFile:572)
at ja.c(SourceFile:67)
at ja.b(SourceFile:81)
at eh.c(SourceFile:319)
at eh.a(SourceFile:276)
at eh.f(SourceFile:280)
at eh.a(SourceFile:217)
at st.a(SourceFile:50)
at eh.c(SourceFile:189)
at eh.<init>(SourceFile:150)
at en.<init>(SourceFile:25)
at net.minecraft.server.MinecraftServer.a(SourceFile:195)
at net.minecraft.server.MinecraftServer.c(SourceFile:150)
at net.minecraft.server.MinecraftServer.run(SourceFile:279)
at ce.run(SourceFile:417)
I had the same problem. I fixed it by deleting the files the server creates and re-download the server application. After that you can play the server with the settings you want now.
I have tried to replace the level.dat, but the error occured again immediately after the world generated.
Does anyone know how I can fix this?
Hope this helps.
Good luck with your worlds!
Error:
2011-10-24 19:01:28 [SEVERE] Unexpected exception
java.lang.NullPointerException
at en.c(SourceFile:72)
at eh.a(SourceFile:2124)
at jf.d(SourceFile:572)
at ja.c(SourceFile:67)
at ja.b(SourceFile:81)
at eh.c(SourceFile:319)
at eh.a(SourceFile:276)
at eh.f(SourceFile:280)
at eh.a(SourceFile:217)
at st.a(SourceFile:50)
at eh.c(SourceFile:189)
at eh.<init>(SourceFile:150)
at en.<init>(SourceFile:25)
at net.minecraft.server.MinecraftServer.a(SourceFile:195)
at net.minecraft.server.MinecraftServer.c(SourceFile:150)
at net.minecraft.server.MinecraftServer.run(SourceFile:279)
at ce.run(SourceFile:417)
http://www.minecraftforum.net/topic/257477-world-recovery-thread-ill-fix-brokenmissing-worlds/