My guess is that you had Windows installed on the hard drive then added a SSD and put another installation of Windows on the SSD. All the boot management data remained on the hard drive, so when you disconnected it, your PC doesn't have the boot management data so Windows won't boot off the SSD.
The worst case scenario option I would consider is to boot from USB, back up your personal data from your SSD via the command prompt (Shift+F10), then wipe the SSD and install Windows on it.
A less destructive option is to try a startup repair on the SSD via the memory stick but I doubt that startup repair is capable of editing the partition table, creating the boot partition that I suspect you're missing, and adding the necessary boot management data to it.
In any case, back up your personal data first in case whatever strategies you try end up making the problem worse.
This tactic I've successfully used may allow you to back up your Windows installation from the SSD, wipe it, do a new install, then overwrite the new install with the old one:
I have a bit of a problem, and my experiment today gets me a (AFAIK fairly definitive) step further towards fixing said problem. Last year, I had the bright idea of using Windows's own disk mirroring system (disk management > dynamic disk > add mirror) as per Microsoft's instructions for two...
forums.anandtech.com
A more direct approach would be to shrink the main Windows partition on the SSD, add a new boot partition manually with diskpart (there'll be diskpart commands needed to mark that partition as a boot partition, and I don't know what those are), but the last step that I'm not too sure about is how to get the valid boot information into that boot partition. I think bootrec might help with this, but it's a long time since I've seen bootrec actually work:
Describes how to use the Bootrec.exe tool to troubleshoot and repair startup issues in Windows Vista and Windows 7.
support.microsoft.com
