Don't overlook the obvious. Start by checking the fluid level in the
transmission. If the fluid level is good, see if the fluid has a burnt
smell. May be indication of something worse. Changing the fluid prolly won't
restore it now though.
Initially it sounded as though you just lost fourth gear, possibly
clutch came apart and got debris in filter/fluid ports. This may have
eventually cut off the flow to other areas. Best bet is to roll it in or tow
it to a reputable shop and let them pressure test it. Yer prolly not gonna
be able to do anything yourself to restore it. IMHO, that is.