As far as I know a Civic does not even use an inertia switch and besides, an inertia switch will only kill the fuel pump; not the entire electrical system.
I had a problem with a Cadillac having all of the electrical go completely dead a few years ago. Estimates on repair were high. I made sure the battery had 12 volts and with the car in park, I turned the key to Aux, made sure the radio or some indicator was on and then gently started moving wires until the radio or something else came on. My problem was a loose wire on the main fuse block. If you have a bad battery or something blown this will not work. With a dead circuit you could start at the positive battery terminal and check at connections until you see zero volts. With everything dead your problem would likely be between battery ground, the battery, and the main fuse block. Otherwise something would get power. I would still have mechanical items checked but this might save you a tow to the repair shop and the electrical costs. Many repair shops are not suited for electrical problems. Good luck.