Read the FAQS and explanation here: https://www.mangago.me/home/mangatopic/21080691/
This is happening to all users right now, but don't worry, your read list will be restored eventually. They're already working hard to fix it. This site is free and maintained by only a small team, with lots of users and many people having very long lists (hundreds or thousands of titles), and the fix is being done manually, so it’s going to take some time.

What the actual fuck is going on!? I just came back from school and this maintenance thing was the one welcoming me