I'm trying to remember if the front element was individual on the lens, and the little screws in the ring are all that's holding it. I think they are, and if so you can loosen them a little, turn the ring to where it belongs, and re-tighten the screws.
In other words, the silver band with the "Canon Zoom Lens EF....", there's three or four screws in it. Loosen them, turn the whole front element, ring and all, to where they need to be, then tighten the screws. It's possible the screws are already loose, and that's why the front element moved. Wish I could help further, but I sold mine months ago, and can't recall how many screws there are.
Typically the front element in in its own cell, and you can take it off and put it on with no issues. Sometimes they need to be in the correct position for the best IQ, but I doubt it with that lens. Just loosen the screws a bit and see if the whole front cell moves, if it does you're all set.