Unfortunately, without more information we can't really help much.
Many of these issues can be caused by any number of things, and the only way we can tell where to look is with the details of what is happening.
What you've described sounds like a lock-up. Lock-ups are one of the more difficult symptoms to track down the cause for, and often requires intimate knowledge of the components/software involved.
I'd be happy to help, but I can't without more information.