The thermostat you show is the high limit.
There should be another one higher up on the heater. This is the thermal cutoff (fuse)
First I would check that the unit is getting 240 volts.
Try flipping the breaker off/on slowly a couple times. Sometimes you can loose half the line without actually tripping the breaker.
Check the voltage at the wall receptacle
L1 to L2 should be 240 volts
L1 to Neutral and L2 to Neutral, both should be 120 volts.
Unplug the unit and check the wires at the terminal strip in the machine to make sure none are loose or burned out
Check the power at the terminal strip.
Do this with the heater off and on.
Be careful as 240 volts is lethal !!!
If the power is OK, unplug the unit.
Check the operating thermostat on the blower, it should be 0 ohms.
Check the thermal cutoff on the heater, it should be 0 ohms.
Check the high limit on the heater it should be 0 ohms.
If the thermal cutoff is blown (open) replace both it and the high limit.
Do not use the old heater as it may be shorted to ground which can cause it to run unregulated (hot)
The way the heater thermostats work is that if for some reason the heater runs hot the high limit opens and closes to regulate the temperature.
But it's contacts are not that robust so after a while they often weld together the thermal cutoff then blows.
After the repair run the unit on a low heat setting to check it.
If it runs too hot then I would replace the operating thermostat on the blower.
I think you may have already done this.