So, you played Etterna, and suddenly you did something that made your game window go black and you had to restart the game?
This black screen can occur when you change your screen resolution, open programs like Photoshop, or for other system specific reasons. You can demonstrate this by trying the first option.
If you can still hear game sounds, then the issue should be easy to fix. To restore your Etterna window from a black screen, open the Debug Menu (F3), enable VSync, and then disable it again (or vice versa).
Hold F3 on your keyboard, then press 4. If that doesn’t work, try holding F3, then pressing F5 and 4.
If none of these steps worked out for you, then there may be a deeper issue that will require you to restart the game.