Late 2015 27" iMac lost the use of two memory slots. What resets to try?

My faithful iMac may have suffered from a rude shut-down (utility power cut, then restore) in that two of its four memory slots suddenly aren't working. (The computer was protected by an APC battery back-up, but battery power may have quit before I noticed). I've tested all four of its cards in the two slots that work, and they all seem fine.) The two slots that don't work are the top two, closest to the top of the display, in the array. If I put a card in either of those slots, the computer won't start---black screen. The computer has weathered other power outages--longer ones, for sure-- without issue. It may be that this one wasn't involved in creating the problem. I'm just providing data hoping that it may be helpful in solving the problem.

When I first used the computer after the power loss, I got the three deep tone warnings, meaning that something was wrong with memory. I re-seated all four cards repeatedly, with no improvement. I finally tried one 8GB card in the bottom slot, and bingo: working computer. Adding the second 8GB card made it an even better computer. But the computer won't run with any of my cards in the top two slots.

The computer seems fine in all other ways, if Apple Hardware test can be trusted. Is this a familiar scenario to anyone else? Is there a possible cure through some kind of reset?