While I don't know how your's is piped here are some things to check.
Are the radiator valves open?
Remove the vent and see if they heat then.
Sometimes the radiators will sink into the floor and if the were piped without much pitch they could waterlog.
See if the pipe hangers are in place.
Check the trap most of the time the float or the temp. device is stuck if the heater has one of those thermostats with the dial numbers take the top part off you will see a pin make sure it is in the open position. hope this might help.