flashing '?' Folder at startup

Possible Fixes for El Capitan and Later Installations

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

Be sure to backup your files before proceeding if possible.

1. Resetting your Mac’s PRAM and NVRAM
2. Reset the System Management Controller (SMC)
3. Start the computer in Safe Mode, then restart normally. This is slower than a standard startup.
4. Repair the disk by booting the from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volumeentry from the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and returnto the Utility Menu. Restart the computer from the Apple Menu.
5. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button below Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
6. Install Combo Updater Download macOS Sierra 10.12.2 Combo Update and install.
7. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
8. Erase and Install OS X Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the drive (out-dented entry) from the Device list.
3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.
4. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Reinstall OS X and click on the Continuebutton.

Since Disk Utility cannot see your hard drive, it has either failed or the hard drive cable has failed or popped loose from the logic board. So the first thing to try is to open up your Mac and check the hard drive cable and reseat it on the logic board and see if this fixes anything.

Is this the 13" mid-2012 Unibody MBP? If so, there is a known issue with the hard drive cable going bad over time, and if this is the case, it will need to be replaced. Replacement cables and instructions are available from PowerBook Medic and ifixit.

Do you have an external enclosure you can put your boot drive in? If so, see if your Mac will boot from the external. If it does, then that really does point to a bad hard drive cable.

As for the drive itself, is it the original? If so, it is getting pretty long in the tooth and should probably be replaced. If the hard drive cable is bad, there's a good chance that the drive has also been corrupted over time as well as being old.

Good luck!