Not booting? Try Internet Recovery! (before you buy new parts!)
All of a sudden, my Macbook Pro wouldn't boot. I shut it down after a day of regular use with no problems, woke up in the morning, and tried to boot it up. It would start loading, then get stuck, and then display a "No Entry" sign. I looked up the "No Entry symbol", and it seems to correspond to either a corrupted hard drive, a corrupted OS, or a faulty / worn down hard drive cable. So, a problem!
I took out the hard drive (an SSD), put it in an enclosure, and plugged it into another Mac. I could access all the files, everything seemed fine, and it was a fairly new SSD after all, so I ruled out "corrupted hard drive".
Then, I backed up the hard drive, wiped it, and reinstalled OS X using the other computer I had access to (another Mac, running up-to-date OS). I tried booting up again from the cleanly installed OS, but no luck. So I figured a "corrupt OS" wasn't my problem.
To test the cable, I tried booting from another hard drive that I knew booted up, but the MBP got stuck in exactly the same spot. I also ran the Apple Hardware Test, and everything registered as OK, EXCEPT the hard drive wasn't even included in the test - as if it wasn't even there! Then I tried booting from the original SSD drive in its USB enclosure, and it worked! So my conclusion was that there must be some problem with the hard drive cable. On to the repair - and to iFixit for the new cable!
I ordered a hard drive cable, spudger, and some lower case screws (a couple of mine were missing) from iFixit on a Friday. The parts came on Monday! I was impressed.
I replaced the hard drive cable with the brand new one from iFixit. But... it turns out the hard drive cable wasn't the problem! Even after replacing the cable, I was getting the exact same bootup problem as before, no matter what I tried! Whyyyyyyyyy...
Then I read online somewhere that it's often helpful to re-install the EXACT OS the computer shipped with - THIS IS IMPORTANT! So, instead of re-installing OS X with the other computer as I'd done before, I put the original SSD back in the Macbook Pro, and tried to boot up with Recovery Mode. But it still wouldn't work - made sense since the OS was now newer than what shipped with the MBP. So I tried Internet Recovery Mode (Command + Alt + R just after the Mac chime), as this promised to install the OS the Mac shipped with. It took an hour or two to download the original OS (10.6), and then another hour-ish to reinstall, but it worked! I was also able to use the App Store to bring the OS back up to speed, and now my MBP is running better than ever! Hooray!
If you're getting a No Entry sign or otherwise can't boot up, and have tried seemingly everything in terms of reinstalling your OS, back up (or swap out) your hard drive, and try Recovery Mode, and if that doesn't work, try Internet Recovery mode. It seems to work better than making a bootable drive using another computer, since it uses the OS YOUR COMPUTER SHIPPED WITH. This worked a charm for me, and if I'd tried it earlier, I would've saved a little cash. Then again, I would've also missed out on a good repair experience, and would still have an incomplete lower case screw set!
So, moral of the story, diagnose twice, fix once, and find other things to order aside from what you aboslutely need for the repair, so if things don't go as expected it'll still feel worth it! :)