I have a Seeburg MCU3 Prelude - had is repaired a couple of months ago...it worked fine...I have not played in in about a month and the part that actually plays the records won't turn or move from side to side. Does it need to be played on a constant basis to continue to work?

No, but like any machine, it does need lubrication once in a while. You should purchase a manual for it (eBay).
If the clutch has become stuck, it's time to call in a pro.
Are you sure it's in "PLAY" mode? Check to be sure the service switch is in "PLAY."
Does the motor turn?
If yes, try pressing the reject button on the back. Maybe it's stuck thinking it's playing a record in an empty slot (clamp arm will be against the turntable). There is also a manual trip lever just to the right of the tonearm. lift up on it if the motor is turning and see if that triggers it. Go ahead, it won't hurt you.
if the motor itself has frozen up from lack of oil, try putting some light machine oil (NOT 3-in-1 or anything paraffin based) in the oil cups on the side of the motor and letting it sit for a few hours. Then, turn the motor shaft by hand a little. This may free it up. if so, count your blessings and call someone in to give the whole mechanism a proper lubrication or something else will eventually seize up or wear out.
Good luck.