I just saw the same with my phone. In my case, the “sudden shutdown” as you call it (I call it “freeze”) was caused by a somewhat common problem that the phone doesn’t respond to anything other than a long-press (~20s) of the power button…
See the related thread “FP2 Frequent crashes requiring reboot by holding power button >10s” for further details of that problem.
After having it plugged-in for a few minutes (I used my computer screen’s USB port for that), the flashing stopped (LED goes solid red) and the phone started and everything seems normal.
Therefore jaymanu’s reply appears to be correct.