1996 Sable GS Transmission Gears Won't Engage
 

Vehicle has approx. 125,000 miles and I believe still has original transmission.
Trans fluid level is good and fluid is clean.
No trans issues beforehand (no slipping, no hesitation)

Problem happened while driving after put trans in reverse and backing up on a small incline.
It felt like reverse engaged as normal, but when applied throttle it just kind of "spun"
And then none of the gears would engage (the column shift still moves freely between all the gears)

No transmission fluid leaking, no transmission related odors.

Has anyone come across this before?
Any suggestions or direction on what the problem might be?
(presuming that it's not a completely failed transmission)

Thanks in advance!!