Flipped off main breaker for repair, unrelated circuit will not come back on
Conduit run into attic is not hot - how are you verifying that? Try an NCV to see if it's a lost neutral. I'd be looking at the neutral bar, because unless you flipping the giant breaker caused the connection to fail at the load center, wherever you did new work isn't as separate a circuit as you thought. Put it all back to the way it was to help isolate the problem.

I suspect you lost the neutral when you ad-hocked some GFCIs.

each side of 220v breaker reads 120V - it's not the breaker.

Losing both sides of a MWBC at the same time would be highly unusual if the breaker works. Trace the neutral.



You dropped the main... Check for any other GFCIs in the entire house that aren't reset. Especially one that might be hidden somewhere and is just two buttons (no outlet).