Chosen Solution
I got my new yellow iPhone 5c for chrismas less than a week ago and it was working fine until this morning when the whole screen went grey with straight thin white lines across it and i can’t do anything with it.
This is usually due to a screen gone bad or a fault on the board, with both issues usually due to a fall/shock, etc. Unless your iPhone looks like it was dropped, you should be able to have it replaced without problem at the Apple Store, like David said.
Your phone will have a year warranty with Apple, take it to the store and they will replace it.
