Screen has lifted and disconected but light shows phone is still responsive. What can i do?

I have a 5c which I have had for 18months. I noticed when I first got it that the top left corner of the screen was slightly raised and wasn't flush with the rest of the phone like the bottom corners/ I assumed it wouldn't be a problem as it didn't effect the phone in any way other than it was just there and meant that the screen wobbled if I pushed that corner. Today the top corner and half of the screen popped out and im assuming something has become disconnected as there is no response on the screen. however when I press the lock or home buttons I can see that there is a light in th gap that the loosening of the screen has created so the phone still works I guess (this is helpful as I can back it up hopefully) Im assuming the phone is no longer within the warranty, but I haven't dropped it or damaged it, its simply a fault in the phone. If anyone knows if I can do an exchange and get a replacement and how much it would cost, that would be good to know.

on a side note, this is the second 5c ive had as the first one got water damaged but also had the same lifting of the screen in the top corners - I haven't met anyone with the same problem but I know some 5/5s's have had a similar problem.

thanks