904 Transmission Stopped Going Into Gear

The tip-off is reverse
To get this gear to work, the low/reverse band needs to engage, and simultaneously the Hi-drum needs to come on.
To get any forward gear, the rear clutch needs to be operational.
To get any forward automatic upshift, the governor needs to be working.
So for the trans to operate as you describe, either: the fluid level is low, the pump is shot, you have a massive internal fluid leak, or all the seals simultaneously puked.There are a couple of other less likely possibilities, and tests for them.

I'm gonna guess the governor exploded,lol, or is stuck.. and the pressure died.

I recommend a pressure test at the L/R servo, in reverse. You should have 230 to 300 psi there, at about 1600rpm. If you're in the ballpark, this is a good sign.
If you're not in the ballpark, check the pressure at the governor port. Here the pressure should be a tested in drive, and should smoothly increase with speed showing around 15psi@15mph and increasing by about 1psi per 1mph,about. And should peak at about 75 psi at some point above 60 mph.

BTW, when the KD mechanism falls off, the trans should upshift very early, like by 12/15/maybe 18mph already into hi. The KD mechanism delays the upshifts, and increases the line-pressure to prevent slippage.
However, if the PO operated it like that for some time, the clutch packs could have been toasted.