How do I begin receiving e-mail?

I recently upgraded to windows 7 from XP. When I set up my new ATT/Yahoo e-mail all of the existing and new e-mails came up in the in-box. The next time I went into e-mail they were all gone except the ones directly from Yahoo. I have not received any new e-mail since. They are not in spam and no filters are in use. How do I get them back and/or begin receiving new mail?