A radiator that won’t heat up often just has a stuck or broken valve.
Step 1: Check if the thermostatic valve head is stuck — remove the head and turn the pin underneath with pliers.
Step 2: If the valve body is leaking — turn off the heating and drain the radiator slightly.
Step 3: Wrap PTFE tape around the valve threads and retighten.
Step 4: If the valve is completely broken replace it — valves cost around $15.