From your description it sounds like the browser window has been snapped to the top half of the screen using Windows Snap Assist. When you click near the top of the screen or double-click accidentally on the title bar area it can trigger this. Easy fix: double-click on the browser's title bar (the coloured bar at the very top of the browser window) and it should snap back to full screen. Alternatively, grab the title bar with your mouse and drag the window down slightly then back up to the very top edge of the screen until you see the full-screen preview appear, then release. If none of that works press Windows key plus the Up arrow while the browser is selected and it will maximise it completely.