Hi,

Yesterday I had 4 receptacles and a ceiling fan go out without tripping any circuit breakers.
Two weeks ago I plugged an extension cord into one of these receptacles with a large load and the outlets went out but also did not trip any circuits. I removed the extension cord and everything has been fine since.

I opened the receptacles and everything looks normal.

What could this be?

Thanks,
John

Try to identify which breaker protects that circuit. There could be a fault with it. Swapping breakers is not very hard (obviously be careful with power).

It is highly unlikely that it is a bad circuit breaker. This goes for most circumstances like this. Bad breakers are always a go-to response but are rarely the problem.
More likely it is a bad connection at a receptacle. Look for a bad back-stab in the first dead or the last working receptacle on that circuit.