Driveshaft

went to atco thursday night -on the 1st run of the night
I do my burn out and staged with no problems
I left the line - no problem, trans shifts 1st to 2nd no problem
were winding out in 3rd when "BAM" the trans let go,
it jumped up and hit the floor of the car, I thought the driveshaft let go
I lost all forward gears

while sitting on the return road I discovered I had no forward gears but reverse worked fine,
so i drove it up the return road backwards

Anyone have an idea what might of broke -
I'm no trans expert so its got me puzzled
why I got no forward gears while reverse works perfect......

89 firebird formula
700r4 trans - with shift kit and corvette servo
**** servo was installed last week and this was first testing of it *****