Do you have a backup? Did they disappear because you archived them? If so,
yes, you can get them back.
If they disappeared because you synced with a pda or cell phone, check in
the deleted items folder and if they are not there, no, you can't get them
back. If you use a blackberry, see http://www.slipstick.com/calendar/missing_appt.asp for steps to avoid the
problem.