In certain instances, deleting Tandem local storage can reset Tandem if it gets into a weird state (can't log in, can't see windows, can't connect to anything etc.)
- quit Tandem
- open Terminal
rm -rf ~/Library/Application\ Support/Tandemand press Enter
- re-open Tandem
Another option for Windows 10:
On the bottom left of windows10 (Type here to search) entering this: %appdata%
Will take you to the following folder to delete: %appdata%/Tandem/'Local Storage'