Could one of the underlying structures be locked?

It may not be applicable in your case but I've found that Toad (a db development tool) will sometimes aquire locks on things that have been browsed and not let them up until the program is closed, even if the user isn't looking at or using the information any more. This effectivly halts any selects/dml on the table, I can't really say why it happens. It's actually been a while since it happened, maybe the newer freeware release fixed it, but possibly you're experiencing something similar?