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Is there any error code or error message on the console?
How old is the Reebok machine in question?
If your model is the kind with a safety key, make sure it is properly inserted, and that the magnet (most of them are magnetic) has not fallen off of it... also, if the safety key is magnetic, and has been dropped onto a hard surface, try the other safety key if you have a spare... mechanical shocks can demagnetize a safety key, making it useless.
I'm sorry to hear that it is simply, suddenly, not working. Some internal failure may have happened, and left the belt non-moving. If the above doesn't help at all, it may need repairs. Would you be wanting to do the repairs yourself, or to engage a repair pro to open it up, find the problem, and fix it ?