This is really causing me to scratch my head. One of my first floor circuits was working fine up until the afternoon. During the morning all was well, at around 11 am I unplugged my laptop from the circuit and it was working fine. At 1 pm I plugged back in and it wasn't powering any longer. I went out and checked the fuse (yes, old house) but it was fine. I got out my circuit tester and it showed a hot/ground reverse. Astonishing! I did do some work on this circuit about six or seven months ago, added an outlet, but I was very careful; it worked perfectly from power-up. In fact, I went under the house where the new branch had been hooked up at and disconnected it. This didn't fix the problem.

I've read on this site where the loss of a neutral in one circuit can cause odd things like this in another circuit, but I have to tell you, there has been no work done on electrical circuits in my house since that one outlet was added half a year ago. I guess I am stumped, so any suggestions are welcome!

Check all the receptacles on this circuit. You have a bad neutral connection somewhere.
Are the receptacles in your house backstabbed. Meaning are the wires just stuck in the holes in the back of the devices?
If so then this can happen any time and for no reason.