Black Screen at Boot!

Hello,
Tonight our computer started acting really strange. Was being really slow. and then it froze. I shut it off and then restarted it. it then said it needed to check the C drive for something...and then that completed and i had to restart it again manually because it was frozen not doing anythign for a really long time..and now when it starts.. it shows the toshiba screen and then goes to a black screen with a blinking cursor in the upper left corner!! WHAT DO I DO!!?? HELP!!!