Electricians

A physical connection has broken somewhere on this circuit, likely because the high current load of your microwave heated up a loose connection somewhere, it melted or arced, and went bang. Now you have an open circuit.

If I were a gambler, I would predict that you will find that one of the contacts on either an outlet or a light switch, or one of their connecting screw terminals, somewhere along the path of that circuit has exploded. Start by checking those first. It will probably be fairly evident where the smoke was generated. If you find wire nuts anywhere in your search, check beneath those as well.

If you are super unlucky you have a break in the conductors in the Romex somewhere. That's both A) annoying, and B) dangerous, since it'll be a pain in the ass to find, another pain in the ass to rip out and replace, and a fire hazard if you don't. This is less likely, because residential wiring is solid copper and is usually quite robust provided it isn't moved or stabbed with something.

There are probably expensive electrician's tools to trace for broken wires using inductance or something. Me, the last time I had to play search-for-the-break I just made myself some long-ass multimeter probes, disconnected the hot and neutral of that circuit from the breaker box, and ran around probing all the outlets with the meter in continuity mode until I found the one that didn't beep.

Everything that was mentioned in the comments are great. I would just add that if you don’t know what wire it is in the attic or elsewhere, you can use a cable tracker. I’ve found that the one from Harbor Freight albeit it cheap and no lasting forever, works better than the Klein ones if you are wearing headphones. It will be able to track the line through the walls, even if it has a break in it. To echo the other person, the circuit will fail in the weakest point, so ideally the breaker is designed to be that. I’d consider changing your old breakers if one failed to trip.

You need to identify the physical path the circuit takes from the panel, through the outlets to where you had your issue. Odds are the circuit is looped through the receptacles (ie one set of hot/neutral through the top set of screws, one set through the bottom screws), and walk it down the line until you find the loose wire. Get yourself one of the $6 outlet testers and pop it into the toaster receptacle; if it lights up, probably an open neutral, if it doesn't, open hot. Just have to find it. Might have to go exploring in your attic or crawl space to get eyes on the physical cable.