I have a Roper Dryer Model REX3514PQO. The dryer

Sounds like you have a broken drum belt.
If you have to replace your drive belt and if you have a front loading machine.
First you will need to unplug your dryer from the electric.
Remove the front of the machine, this will expose the belt and drum, remove the drum and belt and this will expose the drum rollers, idler arm bracket with pulley, and motor with pulley.
While you have the drum out, I would also recommend that you lubricate the drum rollers at the shaft of the roller with a good grade of machine oil like 3 in One oil. This will save you problem later with the rollers getting noisy, also check your drum rollers for flat spots, if they have flat spots on them this will cause a noise like a thumping type noise. You will need to replace the drum rollers if they have flat spots on them.
If the belt is not broken, then I would check to see if the timer switch is working properly, if not then you may need to replace it.
Good luck,
I hope this helps