904 Transmission Stopped Going Into Gear

Don't bother trying to change fluid at this point. AJ/FormS and TrailBeast are correct. Do the pressure tests. If it has little or erratic pressure, it probably broke the torque converter hub. If it has steady pressure at full spec, the clutch packs are smoked. Usually leaking seals don't affect all gear ranges, and usually they smoke one clutch pack leaving eaither reverse gear working or first gear working. Your trans needs a rebuild, but the pressure tests will help determine why.