If you see several Category tabs (Primary, Promotions, Social, Other) and can find your missing emails in one of these, then all you need to do to disable some/all of these Inbox categories is right-click the header of these Inbox Category tabs and de-select the ones you don’t want. Your emails will then be moved back to the Inboxes (or single Inbox) that you have left selected. If you don’t see the Inbox category tabs or can’t find your missing emails within them, then it is likely a different problem. Hope this works for you?
You can also change this in Settings / Mail / Categories
