I had the same thing happen (email stopped) today. I'm using Cingular as my service provider and a BlackBerry 7130c.
When mine stopped I called tech support. They suggested all the things that are described in the prior posts here. Then after that failed to fix the problem I asked to have the call escalated.
The supervisor was much more knowledable, he asked me a few questions and immediately asked me if I would mind if he put me on hold. After just a few minutes he was back and so was my email. He did a reset procedure of my account on his end and that fixed it.
Hope this helps...