MY COMPUTER IS ALMOST COMLETELY BLACK I CAN BARELY SEE ANYTHING!!!?

OKAY, I CONT REALLY KNOW WHAT HAPPENED BUT I CAN BARELY SEE MY SCREEN ITS NOT EVEN AS BRIGHT AS IT USED TO BE, I HAVE TO BE UNDER A LIGHT TO SEE MY COMPUTER SCREEN NOW... SO CAN YOU PLEASE PLEASE PLEASE HELP ME!?!?!??!?!