Why do grey lines appear

Hi,

Have you dropped it or got it wet at all?

If not then it seems as though there is a problem with either the LCD screen or the connection between the screen and the systemboard.

As your phone is a relatively new model, I suggest that you verify the purchase date and if it is less than 12 months ago the phone will still be covered by the manufacturer's warranty. Consult the documentation that came with the phone as to what you have to do to make a claim for a warranty repair or replacement.