I walked away from my computer for a few minutes and when I came back I had the flashing question mark folder - the computer wasn't communicating with the hard drive. I had recently (one year and two month previously) upgraded the original hard drive to an SSD. I still had the old drive, so I put that back in. Same problem. The MacBook Pro didn't start and I got the flashing question mark.
Maybe the hard drive wasn't the problem. Starting the computer with the SSD as an external hard drive (via a SATA/USB adapter) worked. The computer start and ran fine, with no down time since I'm now back to the current hard drive. Of course it's a little slow and it's not the ideal solution. Since the SSD worked, I next replaced the hard drive cable and installed the SSD internally. Same problem - flashing question mark.
I haven't gone any further with repairs, but I've narrowed it down to three possibilities. The new had drive cable that I installed was bad, I installed it improperly or damaged it during installation, or (dread!) the logic board is faulty and is not talking to the SSD. I don't see how I could have installed the cable wrong, but damage is a possibility. Anyway, I'm wondering if there's any test I could do before repairing/replacing the logic board. Any suggestions?