It was working again until today. The thermostat is a basic Aprilaire
model.

I left it set on 74 and it was 77 when I got there - same as before,
but not so hot, so it must have been working until a few hours ago.

I went through the procedure that worked the other two times. This
time it did not work. The 'stat has an indicator that indicates that
it is calling for cooling. It was flashing "cooling" when I turned it
back on. Then after the minute or two delay, I heard the click that
is made when the compressor starts. But the compressor did not start.

We are renting out this house starting sometime in September, so I
need to make sure it is fixed, so I called my usual repairman.

Thanks for all of the helpful replies and ideas.