Sometimes your calendar might stop synching with Exchange.
Well, maybe I’ve got a solution for you:
If you go to Settings, Applications, Active Services, Calendar Storage and select Clear Data, your calendar might start all over with syncing, but it will synch from now on.

If you happen to have the webmail functioning in your orginization, wipe the android device and setup in syncing again in your phone. Sometimes this will work.

Hope it helps you out a bit.