Treadmill stuck on highest incline....and motor running...but seems stuck...won't come down

If the incline is stuck, it could be a problem as simple as the gearbox on the motor being jammed. Unplug the treadmill remove the motor shroud then remove the incline motor from the treadmill, reconnect the wiring, plug in the treadmill and then try to run the incline motor outside the treadmill. Many times, this will unjam it if that is the problem and then reinstalling it in the treadmill will get it to work. If you have the same problem or it doesn’t run outside the machine and you can hear it try to work, you need to replace the incline motor.
If the incline motor does not move, we need to figure out if the motor is getting voltage. Use a simple multi-meter and set it to AC volts. When you put it into calibration, it tries to calibrate the directions one at a time. Measure across the white and black wires first, then measure across the white and red wires second. If the motor is getting voltage you will read 120VAC (or thereabouts) one one of the measurements. If you have voltage but the motor won't move, you need to replace the motor. . If the incline motor does not move and you have done the voltage test and you do not have any voltage present, there is a problem in the electronics board that supplies the power, you need to replace the board.