All my unassigned tasks vanished overnight

Hi, I’m sorry, losing that many tasks is a nasty situation. I’d first stop making more changes in To Do from any device so the deletion or sync state does not spread further. Then check To Do on the web, Outlook on the web under Tasks, and Deleted Items in the mailbox, because To Do tasks are backed by Exchange data. If you recently moved tasks between lists and deleted a temporary list, also check whether the tasks are still visible from another device that has not synced yet. With that many completed tasks involved, I’d open Microsoft support as soon as possible and ask specifically about task recovery from the mailbox side.