What does an exclamation mark mean when I try to open a photo?

It means that the original image file cannot be accessed.

Are you using iCloud Photo Library? Then there may be a problem with the internet connection to Apple's servers.

It can also mean, that your Photos Library has a corruption or a permissions problem and the files in the library cannot be accessed.

Or some third-party cleaning software may have damaged the library.

What happened, before you first noticed the "!" ?

You can see the photos in Photos, because Photos is storing thumbnails in the library for browsing. But to edit a photo or share it, Photos needs the original image file. Since your library is an iCloud Photo Library, the photos need to be downloaded from iCloud. This may not work, if a firewall or antivirus software is blocking the access to the server, or if you are running out of storage on your Mac.

What does the status line in your Photos Library show? If you select the "Photos" album in the sidebar and scroll to the bottom of the window, you should be seeing a status message. Does it show "Updated just noe"ß Or does it show "Downloading ..." or similar?

Do you have a "referenced" or "managed" library?

If it's a referenced library the link to the source file has been broken.

one common reason for this is user error deleting the originals accidentally or by using Malwear like Clean My Mac or MacKeeper - have you done any clean up or run and cleaning software?

ln