I think both three phase and jack are thinking in similar lines but different areas of concern.+ with just 2 possible circuits affected and no tripped breakers or gfci’s , there are a few things to look for ,
First if 2 circuits usually side by side 2 hot wires with 1 neutral it is called a multi wire branch circuit (2 -120v circuits) the problem is a bad neutral . Multi wire branch circuits should be handle tied or common trip breakers today and the neutrals should be pigtailed but this has not always been code and even if it was correct originally sometimes diy updates of outlets or switch’s the home owner pulls a pigtail and uses backstabs. Under heavy load the neutral opens due to heat and the circuit fails. Once it cools down it starts working again.
With a single circuit 1 breaker a similar thing happens but this time it can be on the hot or neutral and backstabs are usually the cause but not always.
The way to find the root cause of both of these issues is to find the first non working receptacle or device , the failure will be there or the last working one.
Most of the time I find it to be a receptacle I turn the power off pull the first non working and inspect if nothing visible move to the last working one , many times one of the wires shows heat damage because of the bad connection and in some cases a broken or burned off wire , extreme cases part of the receptacle is melted.
If everything looks ok and now works where it was not prior to pulling out usually the back stab made connection and the connections need to be moved to screw terminals (with the breaker off) as it will fail again and become more often for longer and or start a fire in the box.
I use a small screwdriver to release the wires from the back stabs then wrap them around the screw 2/3-3/4 wrap and torque the screw.
This has fixed thousands of problems like you mention.
To cover the last possibility I think @jack mentions if not a mwbc ,
If multiple circuits, more than 2 and any 240v appliances make a difference contact your power company and report a power outage there can be several issues that are much more dangerous and usually need professional power company assistance (even for an electrician) they will check both legs and the neutral and that normally doesn’t cost, but I would be looking at the first 2 items as this is very common and if you search backstabs on this site you will find hundreds of questions just like yours.