The most common cause of the fault you describe is the carbon brushes have worn out.
Less common are a faulty motor or faulty control board.
Unplug the appliance.
Tilt it rearward (have someone hold it for you so you can work under it).
(Dont' lay it on it's back, side or upside down).
Take the carbon brushes off of the motor.
There's one either side, opposite end from the belt.
If your machine has a covered base then you won't be able to do this from underneath.
You'll need to take the rear panel off and take the motor out to access the carbon brushes
If worn out or broken, fit new ones.
If this is the only problem, the machine will run again.
If the brushes are ok, or if new ones don't cure the fault, it's an expensive board and possibly motor, replacement.
Check it out, see what you find and get back to me if you need more help.
Obviously, if you find you cannot do the work yourself, you'll need to get someone in to do the work for you.
Hope this helps