One of the strengths and weaknesses of Pages is that it remembers and saves to the last folder saved too. Pages does not mysteriously delete previously saved files though it may save them in an obscure folder location if not careful.
Open your Trash and see if the file is there — since you did not empty the Trash. If it is, then right-click on it and select Put Back or simply drag it to your Desktop where you will know where it is. Back it up.
If the previous paragraph does not reveal the missing Pages document, then review your Spotlight settings.
Your Big Sur System Preferences > Spotlight > Search Results panel should have Documents and Folders categories selected (at minimum). If it does not you will not find any Pages documents using Spotlight. If those are not already selected, then do so, and then click the Spotlight Privacy panel and drag and drop your Macintosh HD folder onto the Privacy panel. Wait ten seconds and then select it, then click [-] to remove it. That will begin Spotlight indexing. You cannot find anything in Spotlight while it is indexing.
Now, enter the following Spotlight query that will show you all Pages documents that you have modified in the last year:
kind:pages modified:>2023
On Big Sur, this will show you the document name and its last opened date. To locate it in your local filesystem, just press cmd+R to open a new Finder Window with that Pages document selected. Or, you can just press Return to open the document in Pages.