I'd check the output of the ignition switch too. Use a volt meter and check the different positions.
If you can crank the starter, then the fusible link is still good. So is the connection under the dash by the ammeter. If either of those had lost their connection, literally nothing would work. Including the starter.
Your '77 would have a Duraspark II ignition system on it, so it's always possible that the ICM (ignition control module) died an early death. It can happen suddenly unfortunately.
When mine did that, it almost always fired right back up. Turned out to be a bad connection at the ignition coil like Oleguy mentioned in another thread.
Good luck. Assume you got back home ok to get back on the computer, but did that happen under it's own power eventually? Or did you get a tow?
Paul