Montier is blank when i stat up

Question

i have a hp pavilion a705w desktop about 2y old it all went wrong when i connected my external hard drive and the desktop froze so i tried to reboot it and the keyboard and mouse dosent seem to work the screen just stays blank and nothing happen the desktop starts up and makes noise but nothing happens i tried the mouse on a different desktop and it works fine i really don't know what the problem is and i really don't want to spend $259 to fix it so if eny one has eny ideas plz help me