It "ejected"? Or it came loose? If it came loose, a bit of rubber cement (put on as little drops around the perimeter - maybe 4 on each long side and 2 on each short site - very small drops - one on the screen, one on the case at each location, then allowed to dry before you put them together would do. (It's contact cement, so once it dries, putting contact cement to contact cement makes a strong, permanent, bond (which is why you want only a few drops here and there - if the screen ever has to be taken off to repair the phone, you want someone to be able to cut through the drops with a razor knife, not destroy the screen).
About the only other choice is to bring it to a repair shop and pay them to reinstall the screen - which will cost a lot more than a small tube of rubber cement.
(Use a small pointed toothpick to apply the dots of cement.)