2000 Blazer Transmission Problem... No reverse!

[esbstuff]

I have a 00 Blazer with 172K. It runs, drives, shifts PERFECT in all forward gears. It will cruise 70 mph all day long. No check engine light! (it works on gauge check). The transmission fluid is full, looks good, has no metal and just slightly dirty. The blazer looks to have been well taken care of. Overall a real nice ride.

BUT it has no reverse, none. It worked perfect one time and was gone the next... what gives?

One person said shift thingy and the next said the reverse band...