MY CHAMPIONS ARE MISSING

I havent played league for a while now, since season 5 +- I had a lot of champions which i even played them in ranked. in the last weeks i entered once a while and noticed that a lot of the champions i used to play with are now unavaileble and i need to buy them. wtf? why do i need to buy them again? its not one or two. its more them 10 champions if not 20. what can i do to bring back my champions without losing my shit?

please help