98% of the time this is a cable issue. Either sticking (especially if cold out) or adjustment. With the engine off and while it's cold, move your shifter from forward to reverse and notice at the transaxle end if one of the cables springs is collapsed and the shifter (on the trans) only halfway. Clean and spray the crap out of the ends of the cables with WD40 at the transaxle end. They may also need adjusting at both ends (hand shifter and transaxle area) only takes one to be out a little to mess it up. Here'a breakdown of the transaxle just in case it does end up being something other than cables sticking and out of adjustment.
