Lost file/document in iCloud Drive and Pages

There are two types of Pages documents: 1) the default, word processing layout, and 2) the optional, page layout format. If you did not purposely visit the Documents panel and deselect document body, before entering any original text in your document, then you have a word processing format. As I have stated, you cannot remove individual pages via thumbnails in a word processing document type, but you can do this in a page layout document.

The first bullet in the Pages User Guide for Mac entry, Delete pages or sections in Pages on Mac - Apple Support refers to the page layout format document when selecting thumbnails and deleting them. These are not instructions for a word processing style document, and you discovered this the hard way.

The absence of any Revert To version history means you are done, unless you can restore that Pages document from a Time Machine backup prior to your thumbnail removal in that document.

Any advice that you received that you should select thumbnails in a default word processing document and delete them would result in lost content. Even Pages throws up a warning dialog telling you exactly what would happen:

What you can do now in Pages with what you have left of the document open, is File menu > Revert To > Browse all versions. You will be presented with the current document view, and if there is any prior autosaved versions of the document they will appear to the right of it. You may be able to use the up-arrow key to go back in time to a point where your document appears prior to the thumbnail deletion. A date/time reference appears below the right-hand document views and just go back beyond the date that you whacked those thumbnails, then click Restore.