The first step of repairing a steam radiator is to inspect the outside of the unit and make sure the supply valve is fully opened. Next, the unit should be carefully checked for heat and listened to for any unusual sounds.
While radiator valves are durable devices, many are in use for decades, so rust and sediment buildup can cause problems, though a bit of cleaning can sometimes fix the valve. If not, the valve or its handle may reveal signs of wear once removed from the radiator. The radiator vents should be checked and cleaned. If the radiator makes a gurgling sound, one of these components are likely at fault.
If the radiator produces no heat at all and makes no sounds, the problem may lie with the boiler or even the thermometer. The boiler should be checked to see if it's operational; if not, a thermostat inspection may help. If the radiator makes sound but stays cool, one of the air valves may be stuck open, which allows cold air to enter the system and cause water to condensate. Problems with air valves often result in sputtering water or steam leaking into the room. Air valves are fairly easy to remove and replace, and a simple replacement may save the unit.