Hi there. I’m an old electrician. Here’s what I think you should do:
1. Turn off your main breaker so that all power is cut to your panel.
2. Make sure all set screws are tight. Especially the ones on the neutral bar (where all the white wires terminate). Many times a loose connection can cause the neutral to be lost.
3. Turn power back on and see if you have 120 volts at your new breaker with a tester that you know works. That way you are sure the breaker works.
4. If you still don’t have power at your receptacle (plug), then go around and test all plugs to see if any others are out as well. Mark the plugs that are out with a piece of tape. ESPECIALLY look for any tripped GFCI receptacles in the area. If you find a GFCI plug; use a pencil eraser or wood dowel to push the TEST button to trip it and then reset it. If you find a tripped GFCI, then re-test your receptacle.
5. Once you have found all plugs that don’t work, you need to open those up and look for loose connections (with power off). Start with the plug closest to the panel. And look at the receptacles that do work in the immediate vicinity.
6. Put it all back together and hopefully you will have found the problem.
7. If all that doesn’t work, call an electrician. Whatever you do though, please don’t rely on these forums or YouTube to further troubleshoot. Get a pro.
Hope this helps.