Well that could be one of 3 things:
- A #blackscreen issue: Please check the #blackscreenguide.
- A proximity sensor issue: Please recalibrate it.
- If the screen doesn’t just go black but the phone crashes there could be many reasons for that. Although it’s not the exact same issue, troubleshooting is about the same as for random reboots, so please check this support article and the first post here.