Try to start, solenoid clicks but starter won’t engage. Further, the entire electrical system, dash lights, interior lights, etc goes out. Regardless of key position, there is a buzzing, or rapid relay clicking coming from the junction box. Whats up?
We need more information. Does your car respond to battery charging? To a jump start?
Likely the battery, solenoid battery cables or starter. Since cleaning and checking cables is something most people can do for free, I would start there. Remove and clean both ends of each cable.
What year (it does make a difference)?