Lost Reverse in 904. Please help!

Pull the pan and hopfully you will find the rear servo cocked in the bore and missing the lever for the rear band.
You also may find the strut laying in the pan when it comes off.

I'd be tickled to find either one of these, because they can both be easily fixed with the pan off.
If the rear band broke the trans has to come out and be completely diassembled to replace it.

Pull the pan and have a look, as that will tell everything you need to know.