Clogged transmissions filter. Low transmission fluid level. A failing/ leaking B2 piston. A failing B2 would also cause no forward gears, slipping forward gears, reverse gear would work. I would check the level first.Drive the car for a couple of miles until the car reaches operating temperature, stable temperature gauge; check the fluid level on both sides of the dip stick, read both sides at the same time, the lowest reading is the true level.