Java Errors Ubuntu Server 10.04

First off, i hope this is in the right place. It looks like im the first one to ever post in the category...which seems a little odd. Anyways..

I had an old computer laying around that i decided i wanted to make it into a small server(maybe 3-4 people) To play minecraft on. And Beings the computer is kind of old, and slow(by todays standards) i decieded to run Ubuntu Server. I got Ubuntu Server 10.04 installed, JRE 6 installed, and the Minecraft Server.jar file installed. The thing worked like a champ for a couple of days. But this evening i went to start it with

java -Xms1024m -Xmx1024m -jar minecraft_server.jar nogui

like i always do, and it gives me this ongoing error:

at ev.a(SourceFile:38)
at ev.b(SourceFile:48)

And then when you do CTRL+C to stop it, it gives me:

2012-08-21 03:20:49 [INFO] Stopping server
Exception in thread "Thread-3" java.lang.NullPointerException
at net.minecraft.server.MinecraftServer.a(SourceFile: 238)
at net.minecraft.server.MinecraftServer.j(SourceFile: 262)
at eo.run(SourceFile:527)

I cant find anything anywhere on what this error might mean. But im also not sure how to search it, because its so big.

Anyways, Id really appreciate help on this.

Thanks,
xb2003