Before doing anything drastic definitely check the fluid level, for it to be so low on fluid it wont engage into gear there should be a noticeable leak somewhere.
If it is full of fluid its probably one of two issues, one is a broken pump, the pump gears on these can break without warning. The other issue is a stuck pump control flow valve in the pump, both issues will require the removal and some disassembly of the transmission, at this point it would be wise to just rebuilt the whole thing. We build these every week and they have a handful of issues that need to be addressed when they are apart.