HELP! I lost all my photos from 2019 when I updated to the new Lightroom Classic

You were probably using a different catalog, and the one that was converted was not the one you were using with the photos after 2019. You need to find the catalog file you were using and update that. Have your operating system search all disk(s) for files whose name ends with .LRCAT, one of those is the one you want.

The new version converted and or opened the wrong catalog. Search your hard drive for files ending in .lrcat and or in LrC go to the File menu and select, hover over, "Open Recent" and see what is listed. If there is more than One catalog listed open the other and see if youj images are in that one.

Please NOTE that there are NO actual image files in the LrC catalog file. It is just a database file where Records, References, are made to where your images are stored on you hard drive. So you have NOT Lost any images. They are still on your hard drive.

And what is the Version NUMBER of your "light blue version"?

Both icons are 'Blue'!

Screen-clip and post  the report you see from the MENU: Help > System Info

(Use the [Insert Photos] icon above your post to include the screen-clip)

It is very likely that you opened an 'old' catalog that does not have the 2019 photos referenced.

Search your computer with a File Browser for all Catalog files that have the .LRCAT file-type,

or in Lr-Classic go- File > Open Recent and see if LrC can find other Catalogs that are more recent.