Suddenly the vehicle will not start?

Ran vehicle to the store when I came out my vehicle wouldn't start. Battery had a good volt reading on the gauge bit when I went to position 4 to turn over the motor, my battery gauge went right to 0v. I've replaced the starter, solenoid and have good cables
on all the grounds and positive as well. I do hear the solenoid engaging. I've tried starting with starter disconnected Ted and battery gauge drops but it doesn't stay drained. When I have the alternator and starter hooked up it goes drained and I have to disconnect the battery positive and reconnect it to try starting it again. Would love some advice if you can think of anything. Thanks