Was it like a
"white noise" screen? I got that once, after using the phone for about a week. I was talking on the phone while it was charging (from a regular electrical outlet), and it got quite overheated. When I was done talking and put the phone down, it looked like that. Had to google how to soft reset the phone, which thankfully helped.
Hasn't happened to me since, but I'm having regular (pretty much daily) problems with the phone just restarting - typically that I take it up after not having used it for a while, and the screen is all black just saying "Nokia" in white text and nothing happens. Have to soft reset the phone every time. Also, from time to time, it will prompt me to enter my PIN-code. Weird indeed.