Photos Icon missing

Photos is a part of the system and protected by the System Integrity protection. It is nearly impossible to delete or uninstall the Photos.app. Open the Applications folder and drag Photos back to the Dock to add a link.

Or are you looking for iPhoto and not Photos? iPhoto can no longer run, if you upgraded the Mac to macOS 10.15 Catalina or macOS 11 Big Sur. On the newer system versions you need to switch from iPhoto to a compatible application, for example Photos.

Where has it disappeared from?

Hit cmd-space. Type photos into the search bar.

Does it find the photos app? If so, just drag it to the dock.

When you drag the app to the dock it will create an icon on the dock. Or are you saying the icon it creates when you do that is blank?

If the latter - are the icons also missing in the applications folder? If so I found this thread:

https://discussions.apple.com/thread/8204906

Which suggests the following:

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If it's both in the Dock and your Applications folder then reset launch services database:

Launch Terminal in your Utilities folder.

Triple click anywhere on the following line to copy entire line. Then, at the prompt copy & paste this command:

Then press return. Wait for the cursor to return to the command line.

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain u -domain s -domain l -v