You might check to see if the email account needs to be validated. All email accounts like Comcast, Hotmail, etc have to be re-validated at least once a year. For more information, see this KB article from Blackberry at http://www.blackberry.com/btsc/search.do?cmd=displayKC&docType=kc&externalId=KB13137
Hope this helps,
Periodically, I lose my emails, too. A local tech found that my password keeps getting dropped and must be input again. I have had my Blackberry Bold for only a few months but this happened 3 times.