It doesn't work at all
If your dishwasher doesn't work at all, check these:
Power from the house
Is there power getting to the dishwasher? This can be difficult to test, because dishwashers are usually wired directly to an electrical circuit, rather than plugged into the wall. But at least check for a blown fuse or a tripped circuit breaker.
The door switch is important because, if it's defective, the dishwasher won't work at all. To reach it, you need to remove the control panel cover, or sometimes the interior liner of the door. If it's defective, you need to completely replace it.
Often the main wiring connection from the house, at the dishwasher, burns and breaks the connection. If that's the problem, the wires must be repaired/replaced as necessary. If that's not the problem, check the wiring to the door switch, the wiring between the door and the lower part of the dishwasher, and the wiring to the timer. Repair any broken, burned, brittle, or defective wiring, using the proper appliance-grade wire and connectors.
Timer or selector switch
Although it's uncommon, the timer or the selector switch may be defective. If so, you need to replace the defective part, because neither of these can be repaired.