This happened with me exactly the same way.
render just stops, counter keeps counting down to zero, elapsed time keeps running, no error message, pc is not frozen, but cancel rendering does nothing.
once the rendering stopped at 80% complete, 70% complete and once at50%complete.
It’s not a pc issue. intel i7 3930, 32 GB ram, GTX680
cpu never gets over 35 deg C because of water cooled.
BTW, this is a V12 issue, it never happened with V10, 11