Hi
Photos is a part of the MacOS operating system. Have you recently upgraded to Catalina, or Big Sur. If so, the old iPhotos app won't run on those versions.
Before following the instructions below, please make sure you have an up to date backup of your iPhoto library (NOT the app, the library). The upgrade can (rarely) go wrong and if it does you need to have a backup to recover from.
What you need to do is open the Photos app WHILE holding the option key. (you can find it in LaunchPad - Icon as shown below.
You will get a pane offering a selection of libraries to open. If your iPhoto library is there, open it. It will be converted to the Photos format (this may take some time) and then you should be able to access all your photos in the new app.