Checks are only valid generally for six months from the date posted on the check. After that, they become "stale" and are not supposed to be honored.
I would be much more concerned, frankly, about the check being in the hands of someone who now knows your bank, your home address, and your checking account number. The ugly, pain-in-the-tukuss extreme is to close that account, unless you think perhaps the check might have been destroyed, eg in a washing machine (left in a pocket) or bundled up in some trash making its discovery somewhat less likely.
Stop payment is the best short-term solution insofar as it at least mitigates the risk from the $100 check, but if you really don't know what happened to it I would start monitoring my credit report very carefully for suspicious activity. Hate to sound paranoid, but having gone through a minor hassle with some fraudulent credit cards a while back, can only offer that being diligent is your best defense.
Good luck.