A few months ago I would have suggested that you download a Windows 7 iso image from Digital River. Unfortunately, Microsoft recently changed their digital distribution and the images from them no longer work with OEM keys.
Your HDD is either dead or the SRP (system reserved partition) is gone so the bios cannot find a bootable device. You may need to buy a new HDD. You can download a HDD test program from the HDD manufacturer support website to test the drive.
If you did not create any backup disks or have an install disk you will need to obtain one -- usually the cheapest Windows 7 install disks are on EBay, just be sure to use a legitimate seller (one with lots of positive recommendations).