Missing Cards from my old collection

I have not played HS for several years and just came back a few weeks ago focusing on Standard and Battlegrounds at first. As I wanted to build a new wild deck I realized that a lot of cards I used to own are now shown as missing, e.g. Northshire Clerik that was still in one of my old decks, but the deck is now shown as incomplete. Any suggestions what might have happened and what I could do?