Hi. Sounds a bit like the motor has a short circuit in the armature. How old is the machine? How old are the batteries?
Remove one or both of the brushes, and try to smell the inside of the motor. A burnt out motor will have a strong smell of burnt varnish.
Is the machine still under warranty?
Another cause could be a seized or binding brake assembly. With the free-wheel lever released, can you easily push the machine along the ground?
Good luck. Neil.