This could be due to a couple of different things, young Wizard. If the only quests you have in your quest journal do not qualify for the Quest Helper, it will not appear. When you do pick up a quest that qualifies, it should reappear.
There's a hotkey to turn the Quest Helper on and off easily and that is the letter T on your keyboard. If you're inadvertantly pressing T, you're turning the Quest Helper on and off.
If you do not have a quest selected in your Quest Journal, your Quest Helper may not appear.
