Did you use bridge or did you change wifi locations? Do you get a 'cannot connect to server error?' If 'no' on all of these and a reboot didn't fix it, it may be easiest to delete and reload you email account. There a number of threads on this topic, including description of suspected/know bugs that may cause this. If you reload your account you may want to only sync the last day so you don't have to open all your previously read messages.
