Is it an IDE or SATA HDD??
Can you try another SATA cable and/or SATA port for starters? This would be the easiest thing to try first of all.
stick the drive in another computer and do a chkdsk on it?
That's very strangeIs the HDD being seen by the BIOS ok and have you set the boot order correctly so the HDD is the first device your system try's to boot from?
There's an easier option you could try - boot from your Windows 7 disk again but select 'repair your computer' this time. When you select this you will be given a menu and one of the options is 'Fix problems that prevent windows from starting up'. Try this.
By the way... if you want to try and repair the MBR.... http://support.microsoft.com/kb/927392