This requires some degree of technical expertise and shouldn't be attempted by anyone who's unsure or not confident in what they're doing. The first and most important part is to unplug the treadmill from the outlet. Then you need to remove the motor cover. You'll need a screwdriver and there should be a few screws either side of the motor and then remove the cover to reveal the motor.
Now the motor is available to you, you need to locate the 2 wires which connect to the power supply. You need to disconnect these. This is why you need some electrical expertise ideally to undertake this procedure. Using a battery, like a car or cordless drill, connect the wires to the battery briefly to check the motor turns.
It doesn't matter if you connect the power wires the wrong way the motor will just run backwards. Don't leave the power connected from the battery for too long. This is just a check to see that the motor is not the problem if you have a fault. If the motor doesn't turn then it is likely it will need to be replaced.
When you're done replace the motor and the wires and components. Don't leave them open to the dirt and grime that could collect on them. When you're organizing the repair or replacement, let the technician know on the phone and when they arrive the steps you've taken. Then the engineer can do the repair for you and your treadmill will begin working again.