You will need to remove the rear cover so you can see inside. Your resistance is either from a magnet or an alternator of some kind. If it is an alternator you would need to check the brushes.
If it is a magnet. There should be a string connected to the magnet and the other side to a little motor with pulley.
The string may have broke, The motor may be bad, or the upper control is not sending a signal.
If you need more help e-mail me
markepeterson@austin.rr.com
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix gym/fitness equipment.