The "T" key is the hot key to turn Quest Helper on and off - alot of times if you forget to hit the enter key before typing a message, you will inadvertently turn it off. Also, it just seems to go off on its own for some people. Alternatively, you can open the Spellbook, go to the Options page and turn it on there. If it still won't come on, perhaps you need to open the Quests page and select a quest for it to point to.
Why would you even need a hot-key to turn off quest helper?
Because some quests don't allow QuestHelper. By default, it will move to a quest that DOES allow it. That can be rather annoying. Also, I have known several people who simply don't like it. They might point out the fact that sometimes, the arrow will point to a teleport stone in the totally wrong area. I've experienced this problem myself, blindly following the arrow without reading the quest and ending up in some remote part of the Spiral which has nothing to do with my quest.