Where have they gone????
Where do you keep your Photos Library stored? Your photos will be in your Photos Library. The Photos Library is by default in your Pictures folder. If your library has been on an external drive, check if your external drive is connected to your Mac and the file system format is compatible (MacOS Extended (Journaled)). Photos will not work with a Photos Library, if the drive, where you are keeping the library, has been used by Time Machine.
See: Move your Photos library to save space on your Mac - Apple Support
If Photos opened a different library than you want to use, quit Photos. Then launch it again while holding down the options key ⌥, until the library chooser dialog appears. Then select the correct library.