Heat isn't coming from the radiator even though it should be on?

It may require bleeding
Usually the landlord is responsible for repair to electrical appliances, He has a legal obligation to keep the apartment in reasonable condition and repair.

What kind of radiator is it? Are you on building steam heat? The building’s heat may not be on.

If it is gas, does it have an electric pilot? the pilot igniter may be broken.

If it’s electric, the thermostat may not be working.

It is all the responsibility of the landlord.

If you signed a lease, you probably can’t get your money back. I agree with @Smitha that it is the landlord’s responsibility to fix it. If you don’t get satisfaction, you can call the Department of Consumer Affairs or the Public Advocate of NYC.

It’s also possible that the landlord has the thermostat for the building set lower than you would like. New York apartments tend to be either way too cold or way too hot.

I have one radiator, happens to be the one in my office, that keeps doing that too. If your heat is hot water, then it’s an air lock and if it only worked for a short time, that indicates the air lock is recurring because there is a leak somewhere in the piping to that radiator, and it gets airlocked over and over. You can try opening the bleed valve till a little water comes out, then closing it tightly. Some hot water systems have automatic make-up of water in the boiler. Mine doesn’t. To get my radiator working, I have to open the air bleed valve, then go down in the basement and just barely open the water line to the boiler until enough water runs into the system to force the air out and water begins coming out of the bleed valve. But this is risky. Too much water too quick in a hot boiler, and you can crack the boiler due to thermal shock. In a big building, the landlord should see to having a plumber find and fix the air leak so the radiator stays on.