|Did you make any BIOS changes recently?|
Is your main hard drive designated as the main bootable device in BIOS?
Does your PC/Laptop detect the Hard Drive?
If you are not sure go into your BIOS (usually DEL key/F2 Key/Esc key) and check if it is being detected and it is the designated bootable device.
Also as mavis007 suggested make sure all external devices that could be seen as bootable devices have been ejected, unplugged or removed (e.g. eject CD/DVD. eject Floppy and remove all USB devices).
☺ When everything else fails, read the instructions.