If you replaced the computer with another, it sounds like you will need to transfer over the images from the old computer drive to another. If you just updated your OS and your images are gone, I wonder if you had your database pointed to a different folder?
If you go to preferences > database > location (I think this is the path). Did that change?
We have our DB pointed to external drive and noticed it had changed back to the home folder > documents
This is default path
If yours is different, I would check there first