Mine has done that a couple of times too. Hold the power/lock screen button on top til it presents power options menu. Select restart or power off either one will work. It'll be fine then. It did it once a day about 3 days in a row and hasn't done it for several days now,
Mine does it all the time. I found that if i lock the screen and take either my finger or one of those glasses cleaner rags and wipe the screen with some pressure it goes back to normal. The screen is super sensitive. Dont ever get something on the screen. It will seem like the phone is posessed.