Thanks for reaching out. We are sorry about the trouble with Lightroom Classic. We are here to help!
Could you please share the version of Lightroom Classic 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?
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!