904 Transmission Stopped Going Into Gear

Well i dont want to be the party pooper, but if its been sitting, then likely the seals dried up.
A couple angles here,if you are dropping the pan to look for anything out of place,and see nothing, then adjust bands and replace filter.
If still no go,will need an overhaul.
As a last resort, there is a “snake oil” that sometimes works on stiff seals.
Atp AT-205.
I have used it and on occasion it works.
If its a ripped piston seal,only parts will fix it...