Hi,
Welcome to our community! Happy New Year! 
Do you use an antivirus program or a firewall that might be blocking Unity Hub?
Regarding your old projects, try to click “Open” and select your “missing” project again.
Also make sure not to use any non-ASCII characters in your path. Unity often has problems with Cyrillic (and other non-Latin/-ASCII characters).
See also:
- Forum User Guides : How to mark a topic as solved