Okay I just upgraded my 4 Guage to 0 gauge , me and my buddy both wired our cars, his car for some reason wouldn't start. He thought it was his battery, anyways my car worked fine, about 12 hrs later I go to start my car and it will not start, its not my battery, I checked it and its at about 12.5 volts, I unhooked the main source and still nothing, any ideas of what it could possibly be??? Sensors, relays, fuses etc.

check all fuses. under hood and under dash,

Check the wire for a cut and it grounding out too.

J

Was this just a power wire for amplifiers or a "Big 3" upgrade ?

explain exactly everything you did.