Because you are out of Transmission Fluid!! Been there, done that. Check your transmission fluid, it's probably very low. Add some and you should be good to go. You should still get your transmission checked out by a mechanic though, because you probably have a leak.
3 forward, 1 reverse
A Super Beetle with a manual transmission has four forward gears and one reverse. A Super Beetle with an Autostick transmission has three forward and one reverse.
your problem is the transmission. you need to replace the transmission
If this is an automatic transmission, more than likely the transmission is on it's way to the junkyard. Usually failing transmissions lose reverse first, then the higher forward gears - last will be the lowest forward gears.
A reverse idler gear in a manual transmission. It converts the reverse gears turning direction into the outputshaft to reverse its direction. The transmission can not spin clockwise when the reverse idler is engaged thus it only spins backwards not forward.
The transmission is broken. Take it to a shop.
The forward pump is shot or the throttle has a blown gasket
the forward gear clutch pack is worn out. the only solution is to get the transmission rebult.
The typical turbo 350 has three speeds forward and one reverse.
Your going to have to get your transmission rebuilt or get another put in.
Your forward gears are out in your transmission I recommend you take it to a shop.
Reverse and forward gears