Any chance you accidentally marked your emails as “Junk”, or set them to “Expired”?
To check take a look in your Junk folders.
I’ve had this occur a couple of times when I’ve been viewing emails on my iPhone and accidentally marked all email as Junk, rather than Read.
When I next look on my laptop the Inbox is empty, as Outlook has set them to Expired. I can then see them in my Junk folder with a strike though, which is the font used for expired emails. You can open each one and mark them as unexpired (there are instructions via Google).
Just to add, there is a way to download all emails from the server to your local Inbox, though I can’t remember the exact instructions off the top of my head. I know because I seem to have to do this once or twice a year!