Hi,
For whatever it might be worth, I fear not much, but reading this reminded me, the system I am now on, once, suddenly refused to get past POST; I checked everything I knew to check, including inside, and, it turned out.....two of my 4 RAM sticks had died! Two big ones I got used on ebay, of course. I bought good replacement new ones, popped them in and everything was good again.
But it took hours and hours of troubleshooting on the floor, cause I had no clue ram sticks could go bad.
Edit: just read initial post again, re error messages.....I think it's not dying ram in yr case. But dying ram happens.....I learned the long & hard way. When I found the bad sticks after the usual interminable chronology of testing each stick and removed the two bad ones, system booted, but was naturally slow.
I would do what Bubbaleon suggested as #3: a repair install of the OS. I have no clue what happend to yr boot files. But I would like to know!