Well there is two things I could think of ... 1. There was a java error. I run my own server and it happens to me all the time. Check the console to see. 2. I know of some hack clients where you can crash a server by push a key or doing a command so that could be a reason. Therefor someone might have crashed it, a plugin failed, or a java error.
In the consle is there a java error that comes up? What permission plugin are you using? Do you only have the two ranks op and default? I need more information to help sorry...