Cold or lukewarm water coming out of the hot water tap
I'm inclined to side with nobyman's shower mixing valve failure. A typical issue resulting from the water being turned back on all at once or too quickly and rupturing the rubber and plastic parts of this century's shower valves.

This is most common with ball valves. The best practice, especially with ball valves, is to just crack them to open and wait for the system to fill and return to silent before opening the valve the rest of the way.

Very rare but also happens with basement shut-offs that are quite a distance from the fixtures by creating a water hammer. This is one reason to have individual shut-offs for each and every fixture everywhere.