Spin the motor by hand. Reach under the dishwasher and spin the fan blades on the motor. The access point is usually narrow. Wear gloves.
2. Check your power supply
All dishwashers should have a nearby shutoff, either a switch above the sink or a cord you can unplug under the sink. If the switch is off, turn it back on. Or plug the machine back in. If it doesn't start, check the circuit breaker or fuse at the main electrical panel to make sure it hasn't tripped. The dishwasher should have its own circuit.
1. Spin the motor
Sometimes the motor sticks and won't turn, especially if you haven't used the dishwasher for a while. One sign of this problem is a humming sound without any other action. To get things going again, remove the lower panels and reach under and spin the motor by turning its fan blades by hand . You must turn off the electrical power at your main panel to do this safely. If the blades don't turn freely, call in a service pro to diagnose the problem.
If the blades spin and the dishwasher still doesn't start when you turn the power back on, you're faced with a more complex problem—perhaps a bad switch, a clogged pump or bad wiring. Call a licensed electrician or a service pro to troubleshoot these types of problems.