Sometimes, Pages will refuse to open its files. As long as it believes the file is unreadable, and that might be infinity, the file is unusable, and only Pages can rectify that — but will not. This is the principal reason to be using Time Machine to consistently back up your Mac (including Pages documents) so you can restore this document from an earlier date and keep working in Pages.
If you have installed any anti-virus application or so-called Mac cleaning application, completely remove it per the vendor's instructions and then reboot your Mac. Try opening that Pages document again, but this time, press and hold the shift key while launching Pages before you attempt to open that document. If this makes no difference, then proceed to the next paragraph.
Reboot your MacBook Air in Safe Boot mode, and attempt to open the problematic Pages document. If you can now open it, save it immediately and quit Pages. Reboot normally and do not proceed with subsequent paragraphs. If the unreadable problem persists, reboot normally and proceed with the next paragraph.
If your Pages document is strictly text without other document objects, you may be able to open it in the current version of the free LibreOffice Suite (Apple Silicon version, click the yellow download box), then save that content as a Word 2010 - 365 .docx document. Open that Word document in Pages to create a new Pages document by that name. The original "damaged" Pages document will not be fixed, but now you have made a copy of its contents in a Word document and that can be opened in Pages.