stupid emachine tech support

hello ive been trying to figure out wats wrong with my sisters computer and ive had no luck. i contacted microsoft and they told me to contact emachines. emachines was no help what so ever. they told me to check all my conections and make sure the computer was plugged in to the wall. well no shit thats pretty dam obvious. i'm having trouble booting the os. before it went to the screen with safe mode, command promt and all that. now i start it and it says disk boot failure. i just want to know what happened that it switched to that and how i can fix it.