On the character selection screen, in the bottom left there's a button "addons", click it and see if "load out of date" is checked in the topright corner of this window.
If it isn't, enable it, if it is, check if QuestHelper is actually in the list.
Then post back.
Well thanks for helping, but i had already check that.
Anyway I managed to solve that problem : /qh hardreset ! (i did not try this at first because i had a problem with QH a few weeks ago and /qh hardreset was helpless at that time)