When you click Check Mail, does it appear to check without problems? or is there a error log tab appearing (just beside the Settings tab) if so, likely the account check is failing....
If this account was working fine earlier AND if you made no changes yourself, possibly your firewall updated and is now blocking the mail server that you use to check mail for this account in MWPro?
If all looks OK, and the email account works in your email program on the same PC, probably the best option here is to try deleting the problem email account and re-importing it....
(First though, I recommend you click File>>Export and save a copy of your settings to file for a backup)
Please open Mailwasher Pro and click, Settings>>Accounts, click your account and click, "Remove Account", click, "Yes, remove it", close and restart Mailwasher Pro.
In Mailwasher Pro click, Settings>>Accounts "Import Account" and follow the directions.
If there are no amounts listed, cancel to exit this screen and click, Add Account, choose your account type and add account manually.
Full directions are available for this from here:
Mailwasher Pro >> Help >> Help Topics >> Settings Screen >> Accounts
Settings available here (for most ISP's but not personal domains) if you need them:
http://www.emailsettings.email/