Bad SATA Cable
My MBP mid 09 started taking forever to startup and I noticed that the sleep indicator light wasn't functioning unless I applied some pressure to the underside of the case. One day the computer just refused to startup and started getting kernel panics and the infamous "missing os" folder at startup. After looking up online I found that this particular model of MBP has issues with bad SATA Cables, I suspected a different reason other than the hard drive, as I managed to start the laptop once, checked the drive and everything seemed fine.
It was super easy following the ifixit guides, the part came nicely packed and even though it was labeled as "used" on the website, it looked brand new. After installing it, the computer came back to life and working great!
Don't be afraid to try your own fixes, in the end the repair costed me something like 50 dollars. I'm sure it would have been much more than that at an apple store!