The close button is the X in the upper right corner on desktop, on the left-hand side on mobile.
Refresh your browser. If you loaded a save while any of the game menus were open, it messes with the controls for the close button and causes it to disappear. Refreshing the page is the current fix.
If the problem persists, then it may be an issue with a browser extension or your browser itself and you may need to experiment with using a different one.