Drive motor is shorted or de-magnetized causing a high power demand.
Check the amp draw using a clamp-on AC volt-amp meter during loaded and no-load conditions and compare values to a similar treadmill that is known to have a good running belt and deck. Replace the belt and/or deck if necessary. Don't forget, some units have a double-sided deck which means it may be flipped over rather than replaced if it has an unused side.
Test unit with a known good drive motor. Replace drive motor if necessary.