How to recover lost works on Pages

When you make a mistake in Pages, and while still in the document, each press of cmd+Z will perform an undo. So, in your case of pasting content instead of copying, a single cmd+Z would have recovered your work.

If you made that mistake and previously had manually saved the Pages document, then autosave would have written that paste into the document before quitting the application. In that case, you would need to use the File menu > Revert To > Browse All Versions… facility on the reopened document. That would let you step back in time to the saved document version before the mistake. Then restore.

Or, you could do the same thing with your Time Machine backup via the Time Machine application. You can use the Finder's Quick Look to visual documents in Time Machine as you go back in time until you see the content represented prior to the mistake. Then you click Restore.