Pictures disappering during update

Please describe more detailed what you've done.

Which previous version did you had installed?

Which new Lightroom version do you have installed?

Were you asked to update the catalog after starting the new installed Lightroom for the first time?

Check if you are in the view "All Photographs" and no filter is applied.

Try resetting the preferences of Lightroom Classic:  How to reset Lightroom Classic preferences (adobe.com)

It's recommended to backup your preferences before you reset the preferences to the default settings: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

Please provide a copy of your 'System Info'. This can be get from the LrC Help > System Info menu item. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.

Do you mean Lightroom Classic?

If so, it has likely chosen the wrong catalog to upgrade. Go to File->Open Recent and try opening each catalog shown there until you find the catalog you want. If that doesn't work, search your hard disk(s) — using Windows Explorer's search function — for all files on all disks whose name ends with .LRCAT, and then open each one of those until you find the catalog you want.