Hydra-Matic issues

I decided to drive my truck to work the other day after a 3 year nap in the garage. It's a 57 Chevy one ton panel with a 265 and hydra-matic. Half way to work (5 miles) it upshifted into nothing. I pulled it back into low, it was good. Back in high, nothing. Drove the rest of the way in low. Went out a couple of hours later to try it again. Low ok, no high gears. Stopped to turn around and it was completely gone. Nothing forward or reverse. I've never worked on a hydra-matic before but it looks like I'm gonna. Anybody want to take a guess what happened so I'll know what to look for. Oh, forgot to mention, ex fire department truck, only 18,000 miles.
Thanks