Hello experts! I live in United States and I really need your help in regards to an issue I'm having with one of my switches. The right-most switch is where the problem occurs. This switch used to turn on a light leading into the basement stairway and is a 1-way switch. The middle and the left-most switches are both 3-way. Before the problem occurred, I could turn on/off the light leading into the basement corridor with either the 3-way switch downstairs or with a 1-way switch upstairs. (image attached) Currently, right-most switch upstairs stopped turning on the light leading into the basement stairway and I want to figure out where the problem occurred. The voltage on the "hot" black wire comes as 120V and there is also a red wire and a bundle of white neutral wires which carry no voltage. My first confusion is how can it be that a 1-way switch and a 3-way switch downstairs both worked together to turn on a light leading into the corridor? Second, if one wire upstairs is the "hot" black wire then how (what instrument) can be used to determine exactly the source of the red traveling wire. In other words, how can I find the other end of the red wire so that I can test for continuity?
P.S. The red and black wires which I indicated in the image below with arrows, are twisted together and only that way a light leading into the upstairs turns on by toggling the middle switch.
P.S. The red and black wires which I indicated in the image below with arrows, are twisted together and only that way a light leading into the upstairs turns on by toggling the middle switch.
