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This may mean that the belt was too tight (whether from shrinkage, wrong adjustment, or lack of lubrication) and that something, either a fuse, or certain elements in the console or in the control board, has or have burned out.
Check for a blown fuse first.
Second, check and adjust the belt:
The solution that you should try first, is that the belt might need readjusting and lubrication (with the special, non-grease, lubricant these treadmill belts generally require.)
The belt should be adjusted, generally speaking, so that it is loose enough that it does not quite slip, but nearly that loose.
If the belt is adjusted (or, has shrunk up with age, and lack of lubricant...) too tightly, then it can stop the treadmill from working or running. An over-tight treadmill belt can cause wear, or even damage, in many other ways, such as by overheating the motor.