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Examine the running belt. Is it frayed or shiny-surfaced?
Is a burning smell coming from the treadmill body?
Is any error code indicated when you press START?
If there is no burning smell, and no error code either, then you may have a simple case of the speed sensor slipping out of alignment. The speed sensor usually 'reads' a small magnet on a rotating element that turns around inside the 'mill when the belt is running. If the magnet falls off, or if the speed sensor comes out of alignment, the 'mill cannot 'read' the movement of the belt, and the belt won't run.
If there is a burning smell, then unfortunately the motor may have burned out - especially if the belt is either worn out and/or too tightly adjusted, etc.
If there is no burning smell and it is not the speed sensor either, the motor controller board may need replacing.
Let me know more about the problem's details, and I will advise you further. Thanks.