You need to pry the lip out in the wheel well so you can smack the top back down,Just use a Large flat bladed screw driver and make sure that the lid is not hung up..
Then go inside and clear away what is needed to get access to the top of the wheel well and use a 12" or 16" 2x6 or 2x4 and lay that on top of the back of the wheel well,, Now whack it back down into position,, If you pried that lip back it should go right back close to where is originally sat, Then staple od screw the lip edge back into the wood backer.
I would then use HVAC aluminum tape and seal up the side and top seams.
You could if you wanted at that point to build sheet aluminum wells and rivet or staple them in,,If you staple with a steel staple,, put a water resistant coating on it,, I have seen Henry's roof patch used for that..
But bottom line,, you gotta get that wheel well back into position!!