As others suggest, my suspicion it will be your crossgate cable going out of adjustment due to linkage wear.
If its the same as the S3 turbo you adjust the crossgate cable, which you will see attached to the gearbox, by disconnecting a clevis pin and rotating the cable end. Mine, presumably through linkage wear, is ultrasensitive to adjustment (i.e if you over adjust for reverse, you will not get 1st - and visa versa). There is only about 1 full turn between these 2 conditions.
I would simply have a fiddle with that adjustment (I can remember whether it is screwed 'in' or 'out' for reverse. You will need to remove the clevis pin, simply try 1/2 turn at a time and see if you can find reverse.
NB I have also seen the crossgate adjustment so badly adjusted that you no longer lift the gearstick for reverse, make sure your box is not adjusted so. (you do lift the lever for reverse dont you!!!).
Good luck
cheers Steve