Info concerning an old AC unit
When an air conditioner is first turned on, the compressor motor encounters high back-pressure from the warm gas, which goes down a bit while running. If nothing is broken in the AC, then perhaps there is a high initial inrush, causing the mains circuit breaker to trip. Try the following:


Make sure all other appliances and lamps on that circuit are off when starting the AC.
If the circuit breaker is less than  20 amperes, try a heavy duty extension cord (rated for more than 15 A)  to an outlet on a 20 A or greater circuit.
If you can remove the AC from its case, see if the fans are free to rotate. Sometimes debris can block them. Lubricate bearing, if possible.


If that does not get it working, then, regrettably, mechanical problems often cost more to fix than the price of a new AC, so there is probably little point in probing further.

BTW, the manufacturer's user manual, p. 24, is less than helpful: "Is There a Problem?
Call the dealer immediately."