My iPhone 4 is frozen on a green/black screen and refuses to do anything!!

So I was charging my iPhone 4 on my computer, and I was transferring some concert videos from my iPhone to my cmoputer to create space. Anyway, about half way through the process of pasting my video's to my computer folder, my iPhone decided to 'time out' and turn black for a good 30-40 seconds. I was frantic and pressed my home button repetitively and tapped my screen etc. and nothing would happen. Eventually my screen came back for a split second, before displaying my screen as a lime-y colour with a black glow on screen's sides and corners. Over about an hour, the lime-y colour has decreased, leaving the black corners to increase and now it is almost covering all of my phones screen! I'm unsure about what I should do from here. What caused this 'green screen of death'? How can I prevent this from happening? Do I have to replace my iPhone? Apple must be entitled to replacing my phone without my warranty coming into play? Will this cost? Will my photos and videos be recovered? PLEASE HELP.