Two things can be wrong, either your transmission has bad transmission fluid in it or has none, and that will tear up your gears and the second is rebuilding your transmission. This is cheaper than a whole new one and you only repair the injured parts. If you are not the only owner then some one else could of done something bad in reverse that screwed this up. One possibilty is that the mechanic tightened your brakes to much as well but it sounds like the transmisson. Take it in to a specialist and have it done.