That would depend on what message is displayed on the computer display when it rejects the bill if a BC1200 model.
You don't give us much to work with here.
Its like bringing in your car to a dealer and saying "my car is broke fix it"
Typical problems with this validator are worn belts, weak optics, bad mag head/amp, bad or weak motor.
Now if you have a C series it can be a little more complicated.
Either way I would recommend either sending it to a Rowe auth repair center such as www.betson.com
or Rowe directly for service.
Tell them you saw their link and Company name on Coinwash.com and too become a Sponsor
If necessary purchase a spare for now and repair the bad one so you have it for that rainy day.
Here are links to the manuals...