Forgive me for I have sinned! I left my '82 spider parked up for about 18 months, with a bad starter. Shame on me!

I replaced the starter this weekend, putting in a good battery from another car, and the car is completely dead, no lights, nothing electrical works at all. The terminals on the batter had 12.5V across them. The heavy duty cable to the starter measured 12.5V. Turning the key, there was no change to the voltage across the batter whatsoever. Any ideas of a likely (and hopefully easy to fix) cause?
thanks,
Phil.