I have run into this very problem for seemingly no reason, and have searched hours and tried too many things to fix it. But i found a fix that worked for me.
Error: I click on the GF client and i get this error in a white box with retry and close as options, same with the Tera icon.
"It appears that the Gameforge Client cannot connect to the Gameforge Client Service"
Solution: Winkey+r type msconfig->services and tick the "Gameforge Client Service" -reboot.
Yeah go figure, of course this service needs to be running but the question is how it ever got shut off in the first place.
Hope this helps someone. 
