How can I fix my Roger Black Treadmill?

I have a Roger Black Treadmill, bought second hand with no instructions. Conmputer still working but after 20 minutes or so treadmill just slowed down, stopped and could not be re-started. Not sure if it is the belt or some other connection. Computer still working now but when start/enter button pressed the treadmill still not starting up. Help!! Many thanks, Julie Lyall