How do you troubleshoot a baseboard heater that is not heating up?
Unless there is a valve in the line that's shut off, you probably either have an air blockage (which either means you need to find a bleeder you haven't yet, possibly hidden under the baseboard cover, or you need to cut the pipe and add one) or you have a frozen pipe (surprisingly common with the way some baseboard systems are installed, especially in older houses.)

If you can bleed air from anywhere else in the system, do that for a start.

Other than "looking carefully at the pipes", feeling them to see where the system is and is not hot can be diagnostic, sometimes. If there's a single pump, and a way to shut off other loops while leaving this loop on the system, that MIGHT burp the bubble out to a place you can bleed it from. If there are multiple pumps, check to be sure the pump in this loop is operating, both by sound/feel and by feeling for heat in the pipes to/from the pump.