Find my untitled document

If it's unsaved it's still in memory. But If you've closed the app and/or shut the Mac down, it's gone. Unsaved is unsaved ... as if it were never there.

If the app is still open, you access it as you would any other in-progress document.

The very first thing you do when you create a new document in Pages, and before you enter any content is to manually save the document with a legitimate (not "untitled") document name. That ensures that Pages automatic autosave is enabled, and changes that you make to the document are saved approximately every ten seconds when working on a local drive, or iCloud Drive. Your content won't be lost when you exit Pages, or it crashes.

However, if you did not follow the first paragraph and never saved your Pages content, quitting Pages causes it to cleanup the "Untitled.pages" file from its temporary storage, and it is gone forever. If however, you do not quit Pages, and switch to another application, then that unsaved Pages content remains, and the first thing you do when switching back to Pages is to manually save that document per the first paragraph.