I have one or two circuits in my home that are not working. I think that it is two separate circuits which both come from a single twin 15A-15A breaker. I believe that three outlets in my garage are on one circuit and two outlets on my front porch are on the other circuit. I removed the outlets and because the outlet which would likely link the two circuits if they were a single circuit was a dead-end I have concluded that it is two separate circuits.
I think that it is likely two circuits that go to that same twin breaker. I went and bought and installed a new breaker, but that did not fix the problem. So then I went and bought both an outlet tester and a 120/240 volt detector. The outlet tester confirms that there is no power to any of the outlets. None of the three lights lit. I went into the panel and tested the voltage on the breaker and found that there is 120 volts on both circuits. FYI, my home is about 20 years old. When I replaced the circuit breaker with equivalent (Square-D), the old one was not terribly corroded and there were no signs of burning. The circuit to my garage had been flipped a few times (months ago) and the 15A is probably not big enough for the air compressor that I occasionally use out there. It seems as though the air compressor can draw too much current when starting during cold weather. I leave the air compressor unplugged, and will add a 20A circuit at some point. As far as I know only the three outlets in the garage and two on the front porch have no power. No lights are affected. I believe, but I should confirm this, that if I flip the breakers off, that no additional outlets are affected.

I realize that just because the voltage detector detects 120 volts at the circuit breaker (I detected by detecting across the screws of the hot wire to the ground in the box) that does not mean that there is enough current to run anything. But there is no voltage at all on the circuits. That seems to tell me that there is a break somewhere. Either a wire or wires broke or came disconnected. I wish that I knew the exact paths of the wires from the breaker box.

Is there anything else that anyone can recommend or have I exhausted what I can do?

I would start by looking for any GFCI receptacles or breakers and reset them.

Do you have a multimeter?

I actually already fixed it and it was caused by a GFCI. For some reason, the garage and front porch were feeding off the outlet in the upstairs bathroom. After writing my original message, I decided to use the outlet tester that I just bought to test every single outlet in my home. I found an outlet in my upstairs bathroom that was also out and of course it was a GFCI. I pushed the reset button and everything worked. I put everything back together and made some notes about my discoveries and am keeping it near my breaker box.

I agree GFCI sounded like the obvious problem. I made the mistake of assuming that my circuits would not be feeding off something upstairs in my home. Thanks.