Photos

From memory, I think one of the options you will have is to create a new Library. Follow that process and if you get the option, designate it as your System Library. The new Library will then download everything from iCloud and you should be in business.

BTW, that's OK if the iCloud Library is up to date and contains everything. An alternative approach would be to use your backups to reinstate the Library you have deleted.

I would check first what Library your Photos app is showing you and whether it is your System Library. It would also help if you provided a few more details, especially anything you did immediately before you lost the photos. Or is it a new MB Air showing you a new Library that is not designated your System Library?

Can you still see your photos on iCloud using web access by logging in to icloud.com? If so, they are still there and safe. You can get them into Photos app by creating a new Library and designating it as your System Library, at which point it will download everything from iCloud (could take a while!).

Give us some more details before you do anything and you'll get more precise advice.