Could be a couple of things. Easiest would be to try a battery pull for 30 seconds or so, then let it reboot after putting battery back in. That re-establishes it on the network. If that doesn't work, you can resend the service books (it re-registers it)(I can't remember the exact sequence, it's all over this forum, just do a search for sending service books)
Lastly, it COULD be a disturbance in the Blackberry Internet Service (BIS). That's happened a few times, and it won't last too long.
But try the battery pull stuff first, and the service books if the battery pull doesn't do the trick. And if none of that works, keep an eye out for BIS service outages...you can also call VZW tech support...they've got some really nice folks in the Blackberry area that will tell you what's going on.
I guess it is time to call. I tried the battery pull several time today and resent the service books. I am not actually having any issues with email, just access the internet. We just got 11" of snow in North Texas, maybe that has something to do with it. That would be about 10" more that we have had the last 3 years.
Try running diagnostics on your phone. Go to Options>Mobile Network. Press the Menu key and select Diagnostics Text. Press the Menu key again and select Run. It takes a couple of minutes to run the report. It might find a problem you can correct.