I began playing this game back when Minecraft.net had some problems and had a 'free weekend' and decided to buy the game after that.  But while it was down and awhile after the server was working fine.  But I took a quick break from it and when I try to turn it on it reads:

[WARNING] Failed to log to server.log
java.io.IOException: Couldn't get lock for server.log
    at java.util.logging.FileHandler.openFiles(Unknown Source)
    at java.util.logging.FileHandler.<init>(Unknown Source)
    at fk.a(SourceFile:47)
    at net.minecraft.server.MinecraftServer.d(SourceFile:90)
    at net.minecraft.server.MinecraftServer.run(SourceFile:186)
    at bm.run(SourceFile:480)

If anyone has any idea to fix this pelase help :(