I see four points of error:
- The heatbed could be faulty.
- The lines could be faulty.
- The trigger signal could be faulty.
- The power supply could be faulty.
So, how to check what it might be? We need to measure, but we need to measure in a setup of 12/24 V circuits while they are live. While not necessarily considered "dangerous", take extra precautions! Remove the parts you will need to access from the machine, ground the power supply, and use your utmost care! Make sure the heatbed is not connected to the Power supply, so we can measure it in the safest way.
Ruling out the Heatbed
The first step can be done without power: the resistance of the heatbed should be non-0, non-open line. It's best to measure directly at the input pins to the bed. If it shows close to 0 (some milli-Ohms), open line (OL), or no continuity (NC), it has a short or is burned out and needs to be replaced.
Ruling out the Power Supply
Once we have reached a safe setup, take a voltmeter and measure the voltage (potential difference) on the pins you reserve for the heatbed. If it is 12 V with some tolerance, it's good, otherwise you might need to get a new power supply or have a professional fix yours.
Ruling out the trigger signal
Next, move your measuring to the output of the board. Now order your board to heat up the bed. Make no illusions, we will trigger TRP, as the bed is disconnected. This will need us to reset the board between tests. You should see a 5 V trigger signal. If not, your board or firmware might be faulty and require different investigations.
Ruing out the lines and switch
Next step is to swap to Resistance mode on the multimeter and measure the resistance of the switch, best with the whole power line. Under "heating", the setup should show a close to 0 Ohm resistance, if not it should show an infinite or open line.
If it is neither of the 4, reassemble on the test stand, reconnect the bed to the power supply and investigate all connections between the elements we just checked. Now order a heating procedure - as all components should be ok, it has to be one of the interfaces between the parts.