Overtightening the walking belt can damage it, so refer to your owner's manual for instructions on where to find the adjustment bolts. Use a wrench to crank the bolts a quarter of a turn, then lift the belt off the walking platform. Once it can be lifted 2 inches to 3 inches, the belt is positioned properly.
Refer to the owner's manual for instructions on removing the hood for the motor. Some machines will require a screwdriver to undo the hood. Others simply lift off. Unplug the machine and remove the hood. Look for burn marks on the motor control board. If you find them, the resistors have burned out and the motor control board must be replaced. Ask the manufacturer about ordering a replacement part.
Center your walking belt with the machine unplugged. Stand in front of the machine with one foot on the floor and the other on the belt. Slide the belt into position with your foot by pushing it backward as you center it. Plug in and turn on the machine at a low speed to check the belt. If it still isn't moving, try turning the adjustment keys until the belt is centered.