Folders missing from Lightroom classic library although not moved

Missing folders/photos issue: I store my photos on my windows laptop hard drive under the file structure Pictures\Year\Camera\Date\photos. I have not knowingly moved them or altered the file structure and the folders and their photos remain in their usual place. Today, many, but curiously not all folders and photos are missing from Lightroom Classic library (a handfull of recent folders and ones from last year are still present). The missing photos cannot be found on a text search of all photos with the lightroom filter bar. If I go to import the missing photos, Lightroom treats them as though it has never seen them. Any ideas before I reimport those that I need to work on?