I bought a 1980 Chevy Monza. All power went out. Would not start and no electrical. Battery is good. What might be the issue?
What engine. A lot pf the Monzas came with Cosworth engines that didn't last. They were hillbilly retrofitted with something else later. Mine had a SB Chevy. 'cause why not? Anyway make sure the engine ground straps were hooked up or the neg battery cable to alternator bracket mod was done and is tight. If it is the ignition switch a toggle switch and push button will have you fast and furious again.
6 cylinder 3.8 auto.
test your ignition switch
Definitely check the ignition switch. On GM cars of this era the switch is not where the key is inserted, it's further down on the steering column under the dash and operated by a pushrod. I had the same no-start/no-power problem on an AMC Hornet years ago and it was the ignition switch. (AMC used GM steering columns.)
Replacements are readily available if needed.

Thank you. Can you tell me if it's easy to get to?
It's been a while but I'm sure it involves some contortions since the switch is on the part of the steering column under the dash.
I ordered the switch, and for the life of me I cannot locate the old switch. I looked under the dash and have no clue. Do you know of any videos or photos out there?
It's been about 25-30 years since I replaced one of those. You may need to remove trim covers from the steering column to access it. This video of repairing a Chevrolet column shows the general location of the ignition switch a little over 5 minutes in:
https://youtu.be/S9YdhqBYZrI?t=325
I ordered the new part, got the old one and attempted to install the new one. There is some kind of push bar that works with the key, not sure where it goes in the new switch, but when I reconnected everything the key would not turn. Also there is a blue tab on the new switch. The old one had a slot where one would have gone. Not sure what it's for.
If you check the video I think it shows where the pushrod goes into the side of the switch. If you can't turn the key the rod is probably jammed into the wrong place. If the new switch doesn't have the same place to insert the pushrod as the old one it's not the correct part.
Also make sure you have good battery cables, good grounds, all connections clean and tight.