As the full message explains, your wallet is already running.
Look in your taskbar tray or the task manager (CTRL+SHIFT+ESC) on Windows, or use top on Linux (the command to view the PID of everything that’s running).
If you can’t get the wallet to respond, you can stop it with the “End Process” option in the Windows task manager or by using “kill -9 [the PID shown in ‘top’]” on Linux.