As stated, easy fix for what may seem to be a bad HD
My MacBook Pro (13" Mid 2012) went blank grey screen of death and occasionally went to the "?" Folder on screen. After going through a few of the HD check steps, realized it did not see the HD. Tested the HD with a USB adapter on a PC and all files were still there. Next check it had to be the HD cable.
Within 15 minutes I was up and running again. Little Mac just experienced a week long coma but is back to normal as if nothing was wrong.
Check your data if you can before just assuming and spending potentially more money and heartache on your HD replacement. Could be the cable... Far less cheaper in time and money!