2000 Chevrolet Truck dead

Hello, I plugged a Bluetooth obd2 scanner into my 2000 C3500 5.7L 2wd and after putting it into drive and going a few feet, the whole truck died. Now it won't start and when the key is turned on, all the elecronics shut off. I am noticing the cab lights are much dimmer than usual and the buzzer under the dash is clicking from not enough electricity getting to it. The battery measures at 12.45v.