I have had a problem similar to this. I was working on a project for a week or so, and suddenly the renders started crashing. It would get to 11% rendered, and then the render time counters would continue incrementing, but the percent of render was frozen at 11%. There were a lot of stills right at the beginning of the project. I finally determined that it was crashing when it reached a particular still. I took that still out, and it started rendering OK again.
I had another project that behaved in a similar manner (stopping at around twenty percent if I remember correctly), but I think I just rebooted on that one and it was OK.