Eight Click on your TERA Launcher shortcut
Click "Open File Location". This will open the exact folder that TERA is installed into."
maybe a solution here
Black Screen - Changed Monitor Resolution In Game
If your monitor has black screened, or powered off after you changed the resolution in the game you will not be able to play until you delete the S1Option.ini file located at c:\Program Files (x86)\TERA\client\s1game\config\s1option.ini. For detailed steps on how to navigate to this file please read below.
1.Click Start, or the Windows Icon button.
3.Double-click the C:Drive.
4.Open the Program Files (x86) folder.
6.Open the Client folder.
7.Double-click S1Game folder.
8.Open the Config folder.
9.Locate S1Option.ini and right-click on it.
11.Load up the game and you should be good to go.