Electrical problems – after the battery has been disconnected, nothing works anymore

Hello Leidensgenossen, A few days ago, the light in the key card slot only blinked, no reaction. At the check of the engine compartment, I noticed a loose minus pole. Pol clamp solved, pole greased and pole clamp again put on and tightened. No reaction, nothing… Since I wanted to expand the display and send in, I did it first. The display is back, but the car seems to be “dead”. What can I do now?