How do I fix a frozen Pages app on MacBook Air?

Press command-option-escape to invoke the Force Quit window. Then select Pages and Force Quit.

Then relaunch tour app and continue on.

The unanswered question is why Pages froze in the first place. You don't help us by omitting the operating system and Pages application versions and what other software you may have recently installed.

Let's hope you had already saved that Pages content once to enable autosave before it froze because if you failed to do that, all content would otherwise be lost.

You’re welcome.