Reinstall the game. When you do, make sure to delete the DBO_CT_TW root folder before you install it again.

There should be either a red X or a circle on every quest you get. If there isn't one then ask some people around if they know how to do the quest. If they don't then just abandon it. No quest in the game is mandatory but they are recommended due to exp. If one seems impossible to do though, then you can always make up that exp by grinding.