Email clients seem to have a nasty habit of losing account settings. The problem is most frequently reported with Microsoft programs (such as Outlook Express) but that's probably only because they're what most people use.
The vast majority of such problems can be solved by simply deleting the account from the email program, and then creating it again. (Deleting the account only removes the connection settings. You won't lose any existing emails!).
Delete your existing account settings (using 'Delete Account' from Tools>Account Settings). Then recreate the account, using these instructions:
http://mail.google.co...swer.py?answer=180189
Chris