If your BlackBerry has stopped receiving incoming messages, it's likely that you need to re-enter your email password. The method for doing this may vary, but it's typically done by clicking on Email Settings. If you don't have an Email Settings icon, try starting your BlackBerry's browser and look for a bookmark for Email Settings. If neither of these work, you'll have to check with your employer (if you have a corporate phone) or your carrier.
If this doesn't solve the problem, try rebooting your BlackBerry by removing the battery.