Logging out and back in is the first thing to do, and a reboot of the computer might not hurt either.
If that doesn't work, why don't you check your logs (via Console in console.log) and see if it reports any files that are giving you difficulty. Specifically, clear your log, launch Activity Monitor, and see what it comes up with in the way of errors once you reproduce the problem. You might have a corrupted or missing file, or you may be running into some sort of conflict with some other software you have. Console should help you sort those issues out, and if you don't know how to read it, just post it here and someone else might be able to help out.