-Harddisk corrupted
-harddsk master boot record corrupted
-Missing OS on boot harddisk
-IDE/SATA cable disconnected from harddisk/motherboard
-Power cable disconnected from harddisk
- You could try repairing the Master boot Record with the Windows 7 DVD (assuming that's your OS)
- You could try repairing the disk and possibly the boot record by introducing a dual boot alongside a Linux OS such as Xubuntu (for speed sake), which would install "GRUB".. a GUI for multi-boot systems that would allow you to see what OSs are installed and choose them at boot time. Then Xubuntu has some software that could help you repair the Windows partition as well.
... the advantage of having a Linux OS dual booting alongside Windows is that you can inspect the Windows partition without hurting it and see what your problem is, and the bonus is you have a fully capable OS that can put you on the internet to seek help while Windows is broken.