IT LIVES AGAIN! 
I held the CUDA button down for 5 seconds just to be safe, and then I took a look at the pins. One was very slightly bent, so I poked it gently with a dull needle to bend it back, which worked. Then I took a look at the rails... and found what looked like a cookie crumb blocking one of them. So the board had to have been out of position as well, which would explain why it didn't look right once I got the screws back on the first time. After removing the crumb the board lined up right.
Just in case anyone is reading this and having trouble getting the board out or back in - be firm! I had to push/pull on the metal holding the board so hard that it sounded like it broke (dramatic banging, for a lack of a better word). But it didn't break. As long as you have the board in the rails lined up right you're OK.
I held the CUDA button down for 5 seconds just to be safe, and then I took a look at the pins. One was very slightly bent, so I poked it gently with a dull needle to bend it back, which worked. Then I took a look at the rails... and found what looked like a cookie crumb blocking one of them. So the board had to have been out of position as well, which would explain why it didn't look right once I got the screws back on the first time. After removing the crumb the board lined up right.
Just in case anyone is reading this and having trouble getting the board out or back in - be firm! I had to push/pull on the metal holding the board so hard that it sounded like it broke (dramatic banging, for a lack of a better word). But it didn't break.
As long as you have the board in the rails lined up right you're OK.