any error? crash? SMP? anything?
On my server, if i wait a while after the hang, it usualy say something, once caused by a world anchor, thehen it was a corrupted ic2 block for me.
Worst case create a backup, and put your region datas in, a fev at a time, so if it is a corruption, then you at least will know where, and you can delete/repair/regen just that part of the map