It sounds like you have either lost the ignition feed to the starter motor or the starter motor solonoid is goosed/sticking.
First try wiggling or removing and refitting the ignition feed. If this doesn't sort it turn the ignition on and make sure the vehicle is out of gear. Get a short length or wire and short the main starter live to the small ignition live. If nothing happens still, Bump start it and get a new starter fitted.