Recently my (automatic) 1996 geo tracker LSI had been slipping a little when it shifted gears. So my father put transmission fluid in it. Turns out, he put in too much and now it it won't move. I'm guessing because it just won't go into gear. I'm not sure.

Any ideas what could be wrong? Thanks!

Too much fluid usually won't keep a trans from working, they usually start
puking fluid from the vents.

What fluid did he use? the biggest problem I have seen is at a trans fluid
change, the new fluid, being a high detergent fluid loosens up crud in the
trans and it gets flushed into the passages and valves causing the problem.

This normally happens with high mileage vehicles that have never or seldom
had the trans fluid changed.

It will probably need to have the pan dropped, and the valve body cleaned
from the bits of dirt and stuff causing the problem, it probably doesn't need
to be rebuilt

Wild

Hmm... That sounds pretty spot on, except that it quit almost instantly. Is it possible I blew a seal or something?

Of coarse it could of blown a seal, you might start by draining
the fluid, or just give it a tear-down and cleaning of the valve
body, or it could be a coincidence, it was starting to act up too

Wild