Did you move any of the photos on your hard disk by any chance?
Tropy creates thumbnails of your photos (which you see in the project view) but opens the original file for display in the item view. If you were able to open a photo originally, the most likely reason is that the photo moved. In that case, you can right click on the photo in question in the photo panel and select
Consolidate Photo; this will allow you to select the original file again. (Note that you have to right click on the individual photo in the photos panel, not in the project table / grid.)
We are working on making Tropy detect missing files and, if possible, resolve their new locations automatically or with minimal user input, but currently you'll have to consolidate each photo individually if you moved it. If these are a lot of photos, and you moved them all to the same location, I could help you make the change directly in your project database (which would be much faster).
If you did not move or rename your photos or the folder they are in then something is obviously wrong. In that case, could you start Tropy, navigate to at least one of the photos which does not show up and then select
Help -> Show Log Files from the menu and post your
project.log here? This will hopefully help us debug the issue.