In the event that the sidebar UI element in Excel gets stuck (appears blank or seems to stay in a loading state forever), you can use the following methods to reset the application cache, based on your version of Excel.
In the desktop version, there should be an information icon in the corner of the sidebar UI:
Click on this to get the menu and then select Clear Web Cache.
Wait a few moments, and from the same menu, select Reload.
In most cases, reloading the frame of the sidebar element will fix the issues with it. The exact steps will depend on your browser, but generally right-clicking in the sidebar and selecting "Reload Frame" from the menu will do it.
For example, the option as seen in the Chrome browser:
If this does not help, you then need to clear the application cache via the developer tools/browser settings. The exact steps to do this will depend on the browser. Then reopen the Supermetrics sidebar.
Example in Chrome browser, you would need to open the developer tools, go to Application and clear these settings: