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.)

Mac:

quit Tandem

open Terminal

type rm -rf ~/Library/Application\ Support/Tandem  and press Enter

re-open Tandem

Windows:

C:\Users\<your user>\AppData\Local\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'

Linux:

~/.config/Tandem