After installing an aftermarket SSD for my MBP, everything was running smoothly. Until one day it just hangs at the gray screen when booting up. After getting a replacement hard drive, swapping out RAM, and attempting to perform Disk Utility, it never booted up. Troubleshot the issue, found out it may be a problem with the SATA cable. So I ordered a new one with a spudger from this site.
First off, very impressed with the service. I ordered on a Tuesday, received the products on a Thursday (I live in CA). Very simple repair, too. Turned out the SATA cable was definitely the issue, as the computer booted up and ran fine afterwards.
Just in case something like this ever happens to you: Always back up your files. One way to be sure that it's the cable and not the disk is to purchase an external SATA-to-USB container for your HDD/SSD. That way, you can run disk utility to see if the disk is the problem. If not, then it's your cable.
Thanks again, iFixit!