I had the problem too, the mainmenu just seems to load slow. The only fix I've found so far was to leave it alone on the 3rd or 4th screen the one with the TERA logo, then when the intro movie starts let it play for a few seconds then enter. Sometimes if you enter too soon you'll still get the black screen but it takes a few second to load in. I'm also pretty sure it's not related to you're computer either cause I'm running tera off a ssd, must be a design flaw.