Need help with this problem with my film camera, ruined my developed roll and soon to be developed since i didnt know it had this problem.

So i just got my film back and all of my shots had this weird black bar on the bottom of each photo, i opened up my camera (after finishing another roll since i didnt know there was a problem with my camera lol) and notice this little gray plastic paper fit the same shape as the shape that came out on my photos, i sliped it back in place but it comes back out slighty which will show in the photos since i did get some with a tiny bit of it showing, how can i fix?Heres the photo via /r/photography http://ift.tt/2tTCFGz