The Blinking Folder of Doom
I'll give it to you short and sweet. I was using my mid 2009 MBP just fine one morning, I shut it down and went to turn it back on a few hours later and it wouldn't boot. Despite trying every key combo in the book all I would get was a blinking folder with a question mark. My first thought was the hard drive had gone bad. I opened the machine and tested the hdd in a nearby desktop and it was just fine. So I knew it had to be the cable or the whole logic board. I was pretty sure it was the cable though since it would boot from the optical drive (I figured it wouldn't be able to do that if the logic board had some major failure). Despite being able to boot from the logic board though, disk utility would not see the hdd. decided to chance it and ordered a new cable.
I fixed it! I guess it was just the cable after all, I've never had a cable fail on me, but I suppose there's a first time for everything. It also seems to be a somewhat frequent problem with my model. Hardest part was plugging in the ir sensor into the new cable, it's very tiny, but apparently pretty robust, I'm surprised I didn't break it.
Follow the guide, it worked for me!