Is the heater hooked up to automation? If so what kind? I would start with the fireman switch and make sure the the toggle switch on the left of heater is on.
Is it a new heater? Are you handy with a multimeter? I would confirm high voltage going in and not missing a phase. Confirm it’s wired according. Than confirm 24vac at the p6 connector. Based on rev the board should have a fuse on it as well. If you don’t feel qualified you should call a qualified technician.
Check the rocker switch. Sometimes they feel like they have are in the "on" position when they are stuck off. Test if power is going in and out, or continuity with the wires disconnected.
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