Hello Emma, the button can definitely be fixed, unfortunately though, it may end up as a costly repair.
The menu buttons are part of a small flexible circuit board that includes your charger port. The flex costs practically nothing.
However to get to your menu keys at the bottom of the phone, the glued-in-place screen will first need to be removed. This is just an unfortunate consequence of how Samsung assembles the phone and how they waterproof it. Removing the screen is risky, and even an experienced technician may tell you there is a 50/50 chance the screen will crack and never turn on again. The screen (part-only) will cost over one hundred dollars to replace.
For now I would certainly suggest backing up your phone's data before you decide on a coarse of action. A local shop may be able to remove the screen without damage, making this an affordable repair. You could also attempt to remove the old screen yourself if you are confident in your own repair abilities. Were you successful in removing the display, the flex you want to replace costs less than a cup of coffee at Starbucks. If screen does crack, you could also order the part yourself and install it at home, saving a bit on what a local repair shop would charge your in labor.