I presume you're referring to the plastic fender liners.  They're attached
at several points to the fenders and bumper cover.  A crack or break isn't
a big deal, but it can allow more road water/road salt into the body areas,
so, depending on how long you're keeping the car and the condition it's
currently in, you may want to address the issue for the long term.  You'll
also want to make sure they're not rubbing the wheels.  The fender liners
themselves are fairly pliable, so you can probably stick them in back
where they belong.  If you don't have the proper fasteners, zip ties may
hold it in place if you're not that concerned about appearance.