Most likely your regulator has died because you used a 12 volt powersupply for extended periods of time and probably feeding secundary circuits and/or components from the 5V Arduino rail. The 5 V regulator gets very hot first and finally dies out because of temperature stress.
Thermal protection circuitry will shut-down the regulator should the junction temperature exceed 165°C at the sense point.
QuoteThermal protection circuitry will shut-down the regulator should the junction temperature exceed 165°C at the sense point.I myself had very recently a AMS1117 stop working and certainly not due to overvoltage, it was fed by a switching 12 volt supply, the current was too high for a long period and the regulator died with a small poof and some smoke.
I suspect the protection in those regulators is meant more for instantaneous over-current than for overheat. So if it's rated 500mA continuous, 2A surge, the safety trips at maybe 2.3A, but there's probably *nothing* in the safety to watch for 1A continuous causing an overheating package. And after awhile, out comes the Magic Smoke.