My phone's screen turns black during use

I have a nexus 5, sometimes during use any phone's screen turns black.sometimes it'll come back after going off and on.sometimes I have to restart my phone to get it working.My screen is not going off,I still can use the screen by touching it but it'll be black.I can play video and stop video or anything else by touching the right place but I can't see anything but a black screen.Someone please help me with this problem..