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The most common cause of the problem you describe is the carbon brushes on the motor have worn out.
There are other causes, such as, broken wires, faulty motor and / or faulty control board.
Unplug the appliance.
Take the rear panel off.
Check the wiring to the motor plug and the motor plug.
If ok, take the motor out and check the carbon brushes on the motor,
If worn out or broken, you'll need to fit new ones.
If all is well, when refitted the machine should work normally again.
If the carbon brushes aren't worn out or broken, the motor and/or control will be at fault.
Click here for a video showing how to change 1 type of carbon brush on a Bosch
It's not your machine but it should give you an idea of the work involved.
You may find that the carbon brushes on your motor are in plastic holders, either side of the motor.
If you get stuck, just let me know and I'll see what else I can do for you.
Obviously, if you cannot do the work yourself, you'll need to call in the engineer.
Check it out, see what you find and get back to me if you need more help.
Hope this helps
Let me now how it goes