Are you running OS9 in the classic environment, or is it just plain old OS9 (in other words, you don't have OSX on the machine)?
If you are running in the classic environment, the problem is likely caused by fonts. If you are running just plain old OS9, its probably a rogue email that is causing the system to quit. You will have to retreive your mail using another computer in order to download the rogue message from your POP mail account. Make sure the message is deleted from the server once it is downloaded, then you should be able to go back and use the system as usual.
Hope this helps.