[hpdad]

There is 2 outlets in the room. I plugged a portable AC into 1. Middle of the night, AC was off. No power on that outlet. The other one is fine. Checked the breaker is fine.

What caused it?

 

[jimn]

A different circuit? It's common to have the outlets in a room on different circuits .

 

[Jim Port]

A failing connection may have opened under the heavy load. Check the last working or first non-working receptacle.

 

[oh'mike]

Before you do any thing--test to see if you lost the power or the neutral.

 

[dmxtothemax]

There is 2 outlets in the room. I plugged a portable AC into 1. Middle of the night, AC was off. No power on that outlet. The other one is fine. Checked the breaker is fine.

What caused it?

Loose/dirty/corroded splice somewhere
Heated up under load and failed.
Have you pulled the receptacle off the wall
look for back stab connections loose slices
or any signs of distress such as over heated wires.

Are you sure there are no hidden gfci's lurking some where ?
it's a very common problem.

 

[hkstroud]

Pull the outlet and see if it was wired using the "Quick Connect" holes in the back.
Use of the quick connect holes is known to develop poor connections after a period of time. Usually about 10 years. The poor connection can be at the non-working outlet or at the last working outlet.

If the quick connect holes were used, remove the wires and reconnect using the terminal screws on the side. Then go around house and do the same for all the other outlets and switches. The problem will keep occurring until you convert all the outlets and switches.