Sync Issue

I was recently on holiday and used LR via my iPad. When I got home I synced it with the Cloud and desktop. There are now a significant number of photos on my desktop LR Classic that have an exclamation mark next to them saying that the file "could not be used because the original file cannot be found. Would you like to locate it?"
If I try to locate it, it navigates to the mobile downloads folder where there are some old photos not relevant.

How do I get the desktop to sync correctly? Plan is up to date and have only used 43.6GB of the 1TB storage.

Paul