Thunerbolt Ports Dead

The Thunderbolt Ports on my 2013 27" iMac running Sierra stopped working. Dead.

They were still listed in the About My Mac system report .. just didn't work. Reboot made no difference. Reset the SMC ( i.e. unplug machine for 15 secs ). Still dead.

Then as a hail mary I reset the P-RAM ( reboot holding option/cmd/P/R ) for 5 chimes. Haven't done that in years ..many years. But it worked! All normal again.