Welcome to the forum and Airstream community!
Look for the main 120-volt circuit breakers, per the location spec'd in your owner's manual. Probably in the front or back of the trailer somewhere.
The lights that went out are probably 12-volt lights, which means you should also look for the 12-volt circuit breakers, which may reset automatically. Do your other 12-volt devices work, like the water pump?
It is possible that your converter (which turns 120 volts AC into 12 volts DC, and charges the batteries) just burned out IMO. If the converter is bad, and your batteries are also bad, or very low, you will not have any 12-volt items functioning normally, but the air conditioner, and 120-volt plugs will still work. The 12-volt distribution panel and circuit breakers can also be located via the owner's manual.
Please plug in a 120-volt appliance like a simple light, or toaster, etc., to see if the outlets work. If they do, and the A/C is still working, at least you have shore power in the trailer.
If you don't have an owner's manual, one can be located online if needed. Please advise.
PS -- This Sovereign
sub-forum may be another good place to ask about locating things in your 1973 31' FYI: