My MBP Mid 2012 shows a question mark in a gray folder (on white screen) when fired-up.
Thought it could be a HD failure. Invested in a brand new Samsung EVO 850 SSD and same problem occurred until I learned about the famous "Flat wire A1278" issue.
After replacement of the flat wire (with felt protection to prevent further damage) and entire recovery, everything was great, super fast booting, app opening in a blink of an eye, me happy.