mobile blacked out and how to fix it

If you haven't dropped it or gotten it wet the problem can probably be fixed with a soft reset. To do this on your phone press and hold both the volume down and power buttons at the same time after at least. 10 seconds the galaxy logo should appear and the phone should be fine.

If that doesn't work, using the original charger and cable plug it in to charge for at least 5 minutes even if it doesn't show anything on the screen. Then try to do the soft reboot again. If that fails you may have a more serious issue requiring professional repair, if it is still under warranty return it.