Battery completely empty – What’s wrong with the car?

Hello everyone, my Clio (1.4 from 2001) does not jump any more after he has been standing for 3.5 weeks. Over Christmas he had already been standing for 3 weeks. In the unusually two weeks in between he started and drove without any problems. When turning the Zuendschluessel nothing does anymore (even no “click” and the bridge with a functional battery did not work yesterday either (not the slightest noise). Can a battery be so empty that the car auc Is it possible to charge the battery overnight with a charger? (Can I easily expand the battery? And if so, how?) Or do I just have to buy a new one directly? Can it be on the battery alone at all, since the car was standing for so long in a row? It was standing and standing in South Africa, where summer is now – so it is not in winter. Or does this point to another car? Because I bought the car used, I’m not sure, but I think that the battery is now almost 2 – 3 years old. Can I just leave the car just like it is until I can care about it? (run on crucks at the moment…) Or is it bad for the car and/or the engine to leave it standing even longer? Thank you for any answer, Anne.