What is the problem and how can I fix it

@ahmeddridi start off with the very first thing I would do. Replace the battery. Then re-evaluate the phone. If it is still not charging, you need to check the charger, charging cable and charging port. Clean the post with a few drops of isopropyl alcohol and few puffs of compressed air. If that does not work, you need to replace the charging port. For now, use the guides from here to replace the battery and the charging port as needed. This model may have issue with the flex cable from the charging board to the main board. I'd replace it.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!