Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, bear with us; re-confirm and post back with the complete model number of the treadmill.
The prescribed standard approach with the treadmill unplugged would be to open up the motor compartment to gain access to the internal mechanism Of particular interest would be the incline motor, shafting and associated gear and mechanism. The only easy user doable try would be to manually turn the incline motor so as to move the incline mechanism even just a bit. The idea is to gently jar the mechanism back into position. Thereafter, try again to apply power to the treadmill and attempt to adjust the incline position.