Google's servers provide the mail, contact updates and whatever else to you on the schedule you decide (in the Settings you tell the phone how often it is to synch with Google for new mail).
Lately, the servers have been reporting that they have nothing for your phone (or mine, or anyone else's). This happens with BlackBerry from time to time as well, and everyone with a BlackBerry suddenly isn't getting new email updates.
I've seen this happen before with Google. It usually lasts only a few minutes. No big deal.