Where do I start when something electrical won't work?

When something won't work and you don't know where to look first, that blank feeling is the real obstacle, not any lack of ability. Every seasoned tech once stood exactly there. What they have that you don't yet is not the answer, but a starting point, and a starting point is a habit you can build. Start where the power starts and follow it toward where it stops. Is there power coming in at all? Yes? Move to the next point along the circuit and check again. Keep going until the power is there on one side and gone on the other. That gap is your fault. You don't have to know where it is at the beginning. You just have to follow the power until it disappears. Next time you freeze, say it out loud: check for power at the source. That one move breaks the paralysis. From there it's just following the trail, one point at a time, until the circuit tells you where it broke. The confidence comes from having a first step, and now you do.