Zero power after new Gfci installed
I would expect this to be a wiring problem, but there is a 99% chance the problem is at one of the ends of wires, i.e. Where they connect to a device like an outlet, switch or splice.    

Code requires that all junction points be accessible in junction boxes.  Almost all junction boxes are behind switches, outlets, lamps, or other points of use.   

So I would shut off the breaker and note what things have lost power.  It is likely a wire connection problem at one of them.  

It almost has to be at the last one which is working or the first one that failed, but that depends on the idea of them being in a "string" one to the next to the next, and it is often difficult to guess where those are.  

The #1 type of wire connection problem is called a "backstab".  We have a lot of Q&amp;A about those.