Reset Yongnuo YN560-II and Canon 580EX II Speedlites
TLDR; Hold down the “Mode” and “Zoom” buttons during startup to do a reset.
I’ve had the Canon 580EX II speedlite for over 15 years. A couple of years ago it just didn’t turn on anymore. Full batteries. No dice. Dead.
Not sure why I kept it, probably because I very rarely ever use flash, so I just forgot to dispose of it.
Today, after putting batteries into my Yongnuo YN560-II, it started up, but immediately turned itself off right after the start sequence.
Ugh. Another dead speedlite? What gives?
Since it did turn on (kinda), it didn’t seem totally broken. So I looked for some sort of reset. After a couple of online searches, I found out that holding down the “Mode” and “Zoom” buttons simultaneously while turning on the flash will trigger a reset.
It worked! The Yongnuo booted up fine.
Now, since I still had the Canon lying around, I tried the same (“Mode” + “Zoom -“).
And lo and behold, it turned on again! 🤯

So if you think your speedlite might be broken/dead, try a reset. It might just be what it needs to come back to life.
(Thought I’d post this here, since searching online didn’t turn up anything straightforward.)