Black screen

Hello,
I have a toshiba computer powered by windows 7. Ive had
It for a little over a year now and its been working just fine until
3 weeks ago. I tried to turn it on and it jumped to a black screen
That show " no bootable device... Insert boot disk and press any key".
It does this everytime I cut it on and the message reappears every
Time I press a key. Now I tried pressing the f12 key on start but it
Still jumps to the same Screen. Then I tried the f2 key and nothing works.
I dont know what to do. I am failin this semester already.because of this.
I really need help. I dont have boot disk by the way.
Help PLEASE !

ow