A little help please.

My 1980 300sd reverse gear just didn't work one day recently. It gave me no clue there was a problem, it just didn't work.

Though sometimes it shifted hard, it has been strong since I bought it 5 months ago.

I realized that the fluid level was high and I am correcting that.

I use this vehicle for everything and could use some guidance here. When this happens is there a neutral safety switch possibility, transmission gone, bands to adjust or a special problem of some kind?

I really need a good place to start here and get mobile again.

Thank You

RAM