Sometimes the partition in question is not recognized by the WinRE (Windows Recovery Environment) or you receive “missing operating system”, “Bootmgr is missing”, or something comparable. In this case, you need to load the startup repair disc. See the link below:
Create Windows 7 Startup Repair Disc
Insert the Startup Repair disc in the drive and press the F8 (most machines) key during boot. This will open the menu to choose which drive to boot from: select the DVD drive on your machine.
Load the Startup Repair disc and select “Next”. Once you have arrived at the window below, select “Command Prompt”
If your Windows 7 installation is not listed, then you need to click on the “Command Prompt” and enter the following commands below:
Type DISKPART at the command prompt to enter into this mode: \’help\’ will list the contents.Next type list disk, ‘Checking Active Partition – Windows 7’