Moving Hard Drives -with OS on them- between computers.

I had temporarily installed an SSD with the OS on it on my newer computer. Now i moved it back to the original computer with the right Key Code . The SSD does not boot the PC. if used as a secondary boot, it recognizes its contents.
What do i need to do to have it to boot Windows 7 Home 64bit once again? (without losing my SSD contents)

Thank you