My Macbook Pro 15" mid-2009 started having issues booting a couple months ago. Symptoms first started as it refused to wake from sleep, and required a hard reboot. It then became worse, freezing right in the middle of my activities, requiring a hard reboot. However, it would come up with the infamous gray screen and flashing question-mark folder. It was intermittent... and very strange. I knew the hard drive was good, since it would work fine when it would work. I also tested it with an external dock. I noticed when I popped the hard drive cable from the logic board, then popped it back in, it would often boot. I figured the cable was bad, and ordered a replacement from iFixit.
The replacement took no more than 5 minutes. I've become quite proficient with removing and replacing the hard drive of late. The new cable worked just fine. However, after sealing it all up, my computer refused to boot. I was at my wits end. I finally took it to a Mac tech. After telling him my plight, he ran tests, and did troubleshooting, like I had. Ultimately, his verdict was that the logic board was bad. Great... $700+ repair, right? Wrong. Additional troubleshooting yielded a discovery: pressing the cable into the logic board while starting up allowed it to boot. Then, a flash of MacGyver-like inspiration: I needed electrical tape, super glue, and... a sock.
I cut small strips out of the sock, wrapped them in electrical tape, and super glued them to the top of the hard drive cable. Screwing down the case applied pressure directly where it was needed. My Macbook Pro now boots! I may not have needed the cable from iFixit after all, but I certainly didn't need a new logic board. Perhaps I will in the future, but for now it's good to go.
Never give up, never surrender!
Keep looking, trying to discover the root cause of your issue. If it's broken, chances are you can fix it with a little know-how, determination, and the right stuff. If all else fails, watch all 7 seasons of MacGyver. There's sure to be something in there to inspire you. ;)