No power to start – what to look at next???

I haven't been on the site in a while and I hope everyone is well. Running into the following problem.

When I first put key into ignition I have power to inside the car. Radio, door buzzer. I turn key to start and I get a loud click and lose all power. After a few minutes power does come back to the interior. I have :

1) replaced the battery
2) checked connections at the battery
3) checked connections at the block
4) checked connection at the relay

Where should I go next? I plan on...

No power to start - what to look at next???