Half the house goes dark — the cause is almost always a tripped breaker. The fix takes 1-2 minutes.
Open the breaker box. Run your hand down the breakers — you'll feel one that's loose and springy, sitting slightly in the middle instead of fully left or right. That's the tripped one.
Push it ALL the way OFF first — hard left, past the resistance — feel click one. Then push it ALL the way ON — hard right, firm pressure — hear click two. If you don't feel both clicks, it didn't reset.
If everything turns back on, you're done. If it trips again immediately, unplug everything on that circuit and try again. Plug things back in one at a time to find the faulty device.
Call a pro if the breaker won't reset, trips instantly, smells burnt, or is hot to the touch.