Could you have [accidentally] activated the "Invert Colors" option? You can check this in your Settings app (General > Accessibility > Invert > Colors). If that's ON, then just turn it off.
Also, to explain how it got activated, it can be set up so that a triple click/press of the Home button can/will activate the Invert Colors. To check to see if that is set up that way, check in the same area (General > Accessibility), but this time, scroll down until you see the line titled "Triple-Click Home." Press that line and you can then see if the trible click function is assigned.
If that's not it, sorry, then I don't know what the problem is.
Marilyn