Yes, I too admin a game server. I agree the RCON password was most likely weak and they just guessed it. OR you have given it out to other admins and they compromised it by trying it out on another server where it was logged and then someone snatched it. I run into that very issue myself. If you didn't give it out then it was sniffed or brute forced etc. Hopefully you have a log or some record of the ip of the bastard who crashed the server. If you don't see repeated attempts as failed log ins, then he has your password through other methods I mentioned at first. If you don't see any logs, then it is perhaps a little more than a scripty or a very stealthy approach and he or she has a vandetta against you. Look at who you banned or flammed recently on the server, see if the IPs cross reference. I know these are a lot of "if"s but that's all I got on the info given.

This is where backups come in handy, if you can't trace how he got in, what he did and be able to see what files where changed, then I would relaod the backup image or rebuild the server. That may be extreme in this case since it just sounds like he crashed the server out of spite (ah, look for logs in game of someone threatening to crash the server,I get those all the time and cross reference) and didn't do any damage, so at a minimum keep a closer eye for a while.

()Peace