Diagnosis
The heating element or its associated safety relay isn't engaging. This is often because the overheat protection (OHP) sensor has tripped due to restricted airflow, but the fan is still allowed to run to cool the unit down.
How to Fix It
1) Unplug the heater. 2) The most common fix is a thorough cleaning. Vacuum the rear air intake grille thoroughly—lint and dust block the sensor. 3) Carefully vacuum the front output grille too. 4) Ensure you have the heater on a hard, flat surface with at least 3 feet of clearance in front and behind. Plug it back in, set it to Heat (not Fan Only), and try again. The sensor needs to see free airflow to allow heat.
If That Didn't Work
The issue could be a faulty connection at the control board. With the unit unplugged and cold, you can remove the few screws on the back panel and check that the wire connectors to the ceramic heating element are snug. Look for any obviously discolored or burnt wires near the element. Don't touch the element itself.
Heads Up
Running the heater on 'Fan Only' mode for extended periods won't cause this, but running it on 'Heat' in a dusty environment or with intake blocked by curtains or furniture will trip the safety. If cleaning doesn't restore heat, the OHP sensor itself may need replacement.