Denise has covered the basics. Look at the voltage, and the wiring first, then the thermostat. If it worked fine and got cold when the compressor ran continuously then it is likely the refrigerant charge is fine. I doubt very much you have to replace the compressor, although if the electronic module is faulty it is possible. If your compressor module has a diagnostic LED, count the flashes and see what it is telling you.
One statement I will disagree with is
Do you know how to handle refrigerant evac and replacement in very small units?" (only one answer; "weigh it in ounces"
A refrigeration tech that REALLY knows what he is doing might charge the system based on suction temp and pressure. In some cases the "factory charge" may not be available, or it may not be accurate because the system line set has been modified.