No reverse??
What transmission are we dealing with... I'll presume a 4L60E for now. How many miles are on it? What sort of life has it seen? What were you doing when you 1st noticed the symptoms.... did you just wake up one day and it was like that?
I suspect that there is a problem in the Reverse Input drum. Whether it broke a snap ring, broke a wave plate etc.... Either way. It is internal and would need to be removed and disassembled for inspection and repair.
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