My photos are greyed out in Photos app and when I click photo to enlarge, I can't see anything

Can you tell us more about your Mac and the photos in your library?

Which system version is running on your Mac? I am asking this, because after the Catalina upgrade some older image formats and video formats are no longer supported. Is your Mac running Catalina?

What is the image format of the photos you cannot open? Are the photos JPEGs or RAW files, or in an older format PICT or BMP?

Where is the Photos Library stored? And have you been running any cleaning applications to remove duplicates or similar? Cleaning applications can damage the library.

All my photos should be in JPG files.

But have you checked this?

Select one of the greyed out photos in Photos, then open the Info with ⌘I and look at the filename in the Info. What is the filename extension?

Can you export the photo with "File > Export > export unmodified Original"? If yes, can Preview open the exported photo?

Or do you get an error message, when trying to export? If yes, what is the error message, literally?

OK - my next suggestion would be to download the trial version of power photos.

https://www.fatcatsoftware.com/powerphotos/

If you use powerphotos to view the library, you can right click on the image, and select "reveal original photo in finder" This should open finder, with the original jpg file selected (if you have a more recent version of photos the filename will not be what you expect, but a long sequence of letters and numbers)

This should show if the original file is still there in the library, or has somehow gone missing. If you open it with preview, you'll be able to confirm it is the correct file.

(Do not try to move or modify any files in these locations, you are looking inside the library structure itself - manual changes will almost certainly corrupt your library)

Can you try selecting the image, then in the image menu select revert to original.