Probable causes.
Boot device has been changed (it happens), especially after a forced shutdown/power cut.
Boot device MBR is corrupted, as above, but also poss virus.
Boot device has had the OS deleted, by accident, or by a virus.
Boot Device isnt working; either dead, or bad cable connection.
Most of these are a free fix if you know what you need to do; if you dont, I suggest you ask the local teenage nerd, or take it to a repair shop.