You see this all the time with dual boot systems and the owner moves the drive with the OS they want to run from a slave or other position to the boot position only to discover that there is no support to get Windows running again.
Yes you can try this or that but all are a BAD IDEA if there are files and more that don't have a backup. I don't want to hand out advice if there is no backup.
What I would do. Install a new blank drive and clone that drive onto the blank. On the clone research Windows 7 REPAIR INSTALL and see if that works.
I had XP and Vista installed on my primary SATA hard drive (on SATA0 channel). There was no space for another operating system, so I added a secondary hard drive (to SATA2 channel) and installed Windows 7 to it. It worked perfectly. After a few weeks my old primary hard drive suddenly died, and the computer doesn't boot anymore. So I put the remaining live hard drive to SATA0 channel, and bought another new SATA drive and put it to SATA2 channel. Now I need to add boot files to my drive, because both operating system and data files are there, I just need to let the computer start.
So I have one good healthy SATA drive, Windows 7 installed on it, but without boot files, because that other (now dead) drive was used to start up the computer. I though that Windows 7 Installation DVD will help, but it doesn't. When I try to use its repair disk feature, it fails to find any installed Windows, so it says there's nothing to boot to, nothing to reapir. If I use the command line utility from the same DVD, I can see both my C: and D: partitions are there, with all files, I can see all my directories, I can see Windows folder, Program Files folder etc. But what does it need to boot?
I also tried "upgrade" from W7 Installation DVD, but it says that I should run this option from inside Windows. Since I cannot start the Windows, this option isn't for me. When I tried a new fresh installation, I got a warning that all my current data will be overwritten, and that also isn't option I am searching for. Any advice?