The first thing to check is that the bath hasn't just tripped the safety thermostat which turns off the controller but leaves the power switch illuminated.
Check the thermostat at the rear of the bath by unscrewing the red cap and press the small red button underneath.
If you hear a click and the controller comes back to life, it's just the thermostat that has been tripped.
This occurs if the bath has run dry and there wasn't enough water covering the element, triggering the safety thermostat.