Hello.
This could be caused by corrupted cache or a JavaScript error.
Start by clearing your browser cache, if using chrome or a chromium based browser you can hit F12 to bring up the developer tools and then right click on the refresh icon next to the address bar. Select empty cache and hard reload, see if the issue persists. If it does you can have a look at the Console tab of the developer tools for errors (they would be marked in red). If any errors appear there they could be caused by a theme or plugin conflict. You could try disabling plugins one by one and refreshing (optimally clearing the cache as well) until you locate the culprit.
