Just plugging it in will usually get you "Nothing". These refrigerators are basically silent and slow in operation. I suggest that you plug it in, make sure that it is level, set the temp for 120VAC to max, put a 1 gallon container of water inside, check it's temp, and close it up. In about 2 hours check the back of the boiler cover for heating. If it's stone cold you will need to check the 120VAC circuits and the heating element. If it's warm to hot, come back tomorrow and check the waters temp.
In the meantime, check on-line and get the installation and operating manuals for the specific refrigerator
If it cools on 120VA it means that the cooling circuit is good and checking on LP gas would be next. If it hasn't been used in a long period or if the flame won't stay ON, read up on periodic cleaning of the vent tubes and the LP gas jet in the manual you just downloaded.
The good news: If it works on any of the three power sources, the other can usually be repaired.
The Bad news, if the cooling circuit has leaked down it's terminal, there is no way to just "Recharge" these and an entire rebuilt cooling circuit may be needed.
BTW: If there is an ammonia smell inside or outside, it's bad news, the cooling circuit has ruptured.
Let us know what happens.