It could be that your home page changed either by an update or by you maybe hitting the wrong key combination when viewing your email ?

Click onto the 3 dots at the top right side of your browser, then scroll down and click onto SETTINGS, on the left side of that page click onto "On Startup", then look for "OPEN a specific page or set of pages". When you list a "specific page", then that becomes your Home page.

See if gmail is listed as your opening page, then delete that address and type something else in. Personally I like having my browser open up to Google so I have http://google.com listed as my "Specific" page.