Had this happen for the first time today, with both iPhoto (which I still like) and Photos, with both a recent-model Canon camera and my iPhone.
After running through the basic options and noticing that there wasn't much current talk about this on the boards (indicating a lower likelihood of a software upgrade issue), I tried plugging the cable from the camera/iPhone directly into the computer and my camera/iPhone were recognized by all the apps as usual.
In my case, there seems to be an issue with my Belkin USB splitter. Old age of the hardware, power surge damage, upgrade compatibility issue...? Who knows. Anyway, will replace. And that direct-plug-in option is another one people might want to try.