Thanks for reaching out. We are sorry about the trouble with Lightroom Classic & Adobe Lightroom. We are here to help!
Could you please share the version of Lightroom Classic & Adobe Lightroom along with the version of the operating system? Please confirm if you are also using Adobe Lightroom on the same machine or any mobile devices? Do you have any VPN or firewall software active which can regulate your computer's internet activity? Are you working with specific videos or images (JPEG or RAW)? Have you tried any troubleshooting steps before?
Please go to Lightroom Classic > Preferences > Lightroom Sync and check if the option for 'Specify location for Lightroom's Synced images' is enabled and if it is, is it targetted to an external drive? If yes, please change it to the internal drive.
In Adobe Lightroom, please go to Preferences > Local Storage and check if the option for 'Store a copy of all original at the specific location' is enabled and if it is, is it targetted to an external dr ve? If yes, please change it to the internal drive.
Please log on to Lightroom Web (https://lightroom.adobe.com) to check current Sync issues on the left part of the screen.
If there are any sync issues, please click on them to know more, find the original for those images, and back them up safely before you empty the Sync issues from Lightroom Web.
Please let us know if this helps!