1. Start+R, paste this: %userprofile%\AppData\Local\Microsoft\Outlook
2. Rename the .pst or .ost file (Hint: the file name will contain your email address) with "-old" at the end, but before the file extension Ex. filename-old.pst or filename-old.ost
3. Fire up Outlook and see if your mail starts downloading.
If not, then you'll have to check the profile settings to ensure that the mail server info is still valid.