My first guess is the float switch. If it really is a mechanical float switch, try unbolting it. Try to repeat the failure by tilting it and returning upright. If you get it to fail see if mechanical action fixes it (shake it).
If it is a solid state switch, then I'd be even more suspicious of the float switch, try
cleaning it then manually tilt it, put it
water,
lift it out. Try to recreate the failure.
John