In reply to Rog Wilko:
Assuming you have checked the items mentioned above ...
Is there more than one disk in the system? so check its trying to boot off the correct disk. You need to do this in the BIOS. Best to look how to do this on t'internet. Essentially press the on button from a cold start and immediately press and keep pressing either the [Del] key or the [F9] key to eneter the BIOS. Check the BOOT settings. Press [F10] to save and reboot.
Do you still have the installation CD or the rescue disk you were supposed to make when installing windows? The boot record maybe corrupt (happened to me a few times). Put in the boot CD and choose the "repair" option.
If you don't have a boot CD there are repair disks available online, just download and burn to a CD or make a bootable USB. Instructions usually on the site. These will have a tool to fix your boot record, amongst many tools.
When doing the BIOS fix above it should list the drives in the machine. If none are listed then the cable may have become loose. Pop the side/top off the case and check the data cables to the drive at both ends.