When I first started reading your description I was thinking you may have reseated incorrectly. You do not have to remove the cable to re-seat it, just rock it up the pins without taking it off the pins then press it back a couple times.
A method I have found that works well for me is to put the cable end at a 90 deg angle to the header pins so I can see them lining up, then tip the cable on the the pins. It is very easy to connect them off a row. Glad you didn't damage anything else! Great story, you are not an idiot, you just did not realize, now you know and you will make sure they are lined up correctly. Live and learn