Won't turn on

If you know how to open the machines and can gain access to the internal terminals and you have any kind of meter with Ohms function, check from each side of AC plug to the terminals inside the machine, obviously, with nothing plugged into the wall.
You should have readings of less than 2-3 ohms for each line and it should be even less.

Then place the switch in the 'on' position (STILL NOT PLUGGED IN!) and measure across the internal terminals.
Depending on the motor and condition of the brushes, you should measure resistance from somewhere between a few ohms to perhaps a 100+ ohms.
If it measures much higher, either the brushes are no longer contacting or a motor winding is open.
The latter case would likely make the machines worthless since most manufacturers will charge way too much for a replacment motor to make it worthwhile.

Remember to avoid having anything plugged into the wall since the procedures described do not require it.