I just deleted something unintentionally. How do I get it back? There is no re-do

The 'Oops! key' (command-Z) will undo many actions,provided you press it immediately after making the error.

If you have Saved the document at least once since creating it, Pages will have saved several versions of the document as you edited it. Try Revert to… in the File menu:

Then browse recent versions to find th most recent one including the material deleted.

Regards,

Barry

What Barry suggested, or Pages is why they invented Time Machine backups, so you can restore a lost file.