On iPhone or iPad, open the Photos app and sign in with your Apple ID
Make sure iCloud Photos is turned on in Settings > [your name] > iCloud > Photos
On Mac, open the Photos app and sign in with your Apple ID
Make sure iCloud Photos is turned on in System Settings > [your name] > iCloud > Photos
On Windows, install iCloud for Windows, sign in, and open iCloud Photos in File Explorer
On a web browser, go to iCloud.com and sign in with your Apple ID
Select Photos on iCloud.com to view your iCloud photos
Check that you are using the same Apple ID on all devices
Make sure your device is connected to the internet
Wait for photos to finish syncing if they are not appearing right away