First of all make sure you are using the correct login info. Remember the username if the full email address for the account.
If for some reason you have turned off the mailbox in the control panel, you will be unable to log in via WebMail. The solution is to log into the control panel and turn on the mailbox. If you are forwarding to another address you can turn the mailbox off (so email will not accumulate) but WebMail will not work.