Non Start
 

I have a 2002 RX300. The car was starting fine but the battery was a little low. I charged the battery overnight. When I went to attach the positive cable the next morning, the horn starting going off (alarm) and would not stop. I removed the horn fuse and trued to start the car. Nothing. No dash lights, no radio, no headlights. Nothing. I tried to use the master key to start it. Nothing. I did the outside lock hold for 30 seconds thing. All that did was lower the drivers side window and now that's stuck open as well as the car not starting. I cleaned the cables and I bought a new battery. I have power coming off the positive cable at 12.50 volts. I have no idea how to check for issues or fix this.