My MTD ride on lawn mowers gears stopeed working then started again and now only 6 gear is working any ideas I would really appreciate the help

You probably need to replace the drive belts. This mower probably uses the two drive belts along with a variable speed pulley to determine the speeds. There is no transmission. Look above the deck to see the upper set of belts. You can also see them if you take the plate off by the shifter.