Mobile and Tablet View Buttons Missing

[ Moderator note: moved to Fixing WordPress. ]

Hi,

In the customiser I used to be able to click on a bar at the bottom and preview what my site would look like on a mobile, tablet or desktop. This bar has disappeared; how do I get it back?!

I’ve kept the same theme which is a responsive theme and it did work previously it just seems to have disappeared.

Thank you.