This is an interesting scenario to say the least. It is possible your OST or current default folder file, PST, has become corrupted or your current email profile. Another solution could be you no longer have your Offline Address Book.

You e-mail account, is it on an exchange server?

Here is information on how to repair a corrupted PST.
http://office.microsoft.com/en-us/ou...001056300.aspx

Please keep me posted.