Actually, this happens every time there's a major patch, almost no matter what addons you're using. It would be best to find where you downloaded the addon from (I'm partial to
Curse.com personally), and download another copy of it. Usually the websites will have updated versions of the addons within a few days of the patches. On Curse.com, it will tell you what patch the specific addon works for, and you download the one for the latest patch, 2.4.3 I believe was the latest one.
Because QuestHelper has to be updated whenever new quests are added, it will need to be replaced nearly every patch, while some addons don't, like AlphaMap (because the map hardly ever changes in WoW, except for stuff like patch 2.4 where they added a new area).
And to make things simple,
here is a link to QuestHelper on Curse.com. Scroll down about 3/4 of the page, and there should be a link for QuestHelper for 2.4.3 to start the download.