very strange electrical issue,

[K20Submarine]

very strange electrical issue, i have an 86 k20 burb 5.7. First instance after sitting in the driveway for 2 months being periodically ran for about 20 mins a week. So today I started it up ran it for a few minutes shut it off installed a new dash speaker and a capacitor to the sound system. Everything with the sound system except for the head unit is on a circuit breaker that was turned off. So I went to start the truck back up and, I got lights and the buzzing noise. The ignition clicked once and all electrical was gone. So I attempted to jump start the battery thinking I had killed the battery from charging the capacitor, But jumping the battery had no affect. So I went and got a multimeter and checked the battery directly about 3 hours after it wouldnt take the jump. The multimeter read 12.6??!!!! I looked into the truck and all the lights were on, so I jumped in and tried to start it, all electrical shut off and would not come back on. What the hell is going on?!!