lsass.exe

[F]

Recently on re-booting I got a message saying there was
a "lsass.exe system error" referring to some invalid
password issue, yet nobody had changed a password and I
was only one in the house so I know nobody had tampered
either.

I could not get into safe mode, nor did going back to
last configuration which worked help. Recovery console
failed too and my PC support said I had to reformat.

My questions are:-
1. What could have caused the problem?
2. What can I do to avoid it happening again?
3. Was there a way to recover without reformatting?

Thanks