This is my first time on this board seeking help on an electrical problem.
Yesterday I was replacing my old porch light fixture. I did not think it was a big deal. Well, it has become a BIG deal. When I tried to to put a new fixture in its place, I noticed the electric wires were dead, no power! What happened? Yes, I did check the breaker in the panel box.

Here is what we have. The electric box on the wall where the porch light was mounted has two sets of electric cables coming to it. The one connected to the light has four wires, White, Black, Red and ground. White and Black were connected to the light. The Red now I don't remember where it was connected to. The second cable also has four wires. The Black is taped and not connected to anything. The Red had a wired nut on it and seemed not connected to anything. The White is connected to the White on the first cable.

These two sets of cables go to a wall switch box that controls this light and a couple of other lights. Below this switch box there is an electric outlet off the floor. The whole thing as described are connected to one circuit breaker in the main electric panel box.

If I haven't confused you and you are still with me, here is the situation now. There is No power in that outlet and no power in the light cable on the porch wall and none of those other lights connected to it work any longer. But there is 120 volts in the second cable going to the porch light box that seemed not connected to anything. The strange thing is that the Red wire in this cable shows 120 between it and any other wires from the first cable! No matter which one (white, black, or red) !

That is where we are and I am afraid to go further without knowing what happened and what to do. Did I disconnect something in that light box by accident when removing the old fixture? Did I short circuit something by accident? Why do we not have power in the light fixture and other lights? Where those Red wires go to? And where do we go from here without causing damage?

Although I am not an electrician, I am an electronic technician and know how to follow lines and take measurements. But here everything is in the wall and we are dealing with high voltage AC. That is why I am here on this board seeking help and guidance from knowledgeable electricians. So, your help would be greatly appreciated. If necessary, I can make a drawing of the wires in the box. Thanks

My panel is full (commander 125A sub panel connected to a 100a house breaker by the meter) It takes Eaton breakers. As the greater needs a 2 pole, do I need to bit a quad to fit into the space of 2 existing breakers and create room for a 2 pole? I have 1 empty 15a breaker. I assume I could also fit a tandem in there but the only tandems I can find are for 2 separate circuits ie each is 1 pole.