SGS2 screen gets more and more dark blue, possibly after overheated charging

My phone screen is gradually becoming dark blue and then black--see photo. Started with a single light blueish edge, and growing during the last hours to what can be seen on photo.  The only serious reason that I could think of is that the phone has recently overheated while charging: it stayed for several hours under the pillow, and it was very hot when I took it away from there. Another guess is a recent drop on the floor, but in its 2-years lifetime it dropped dozens of time, frequently much harder than today. What could be the reason, and how it can be fixed, ideally DIY or at all? It's Samsung Galaxy S II GT-I9100.