iphoto

The exclamation point is indicating an incompatible or unreadable file.

are you perhaps trying to open RAW photos or photos synced from an iPhone with iOS 10.13 or newer?

You may want to check the filename extension of the files with the exclamation point. what does the Info panel show for these files?

If the filename extension is .heic and the images have been synced from an iPhone, you should change the Settings on the iPhone or upgrade to the Mac to MacOS 10.13 or newer as described here: Using HEIF or HEVC media on Apple devices - Apple Support

Are you using Photos or iPhoto? What version?

The exclamation mark is an indication that the link to the original image file has been broken.

Are you running a "managed" or "referenced" library?

Check the General preference pane for the app to see which it is.

What happens, when you try to edit the photo? Are you seeing an error message?

If the photos have been imported as referenced, you should be seeing a yellow badge with a broken arrow, and when you try to edit the photo a prompt to search for the missing file. It would look like this:

The badge overlay in the upper right corner:

And the alert when trying to edit:

If you are not seeing this message, try to export the unmodified original, so you can check the format in the Finder.