Admin bar disappeared from back-end

… and the problem has mysteriously resolved itself, just a few minutes after posting the above – without changing anything on the site.

However, I would still like some pointers as to what may have caused this – as there’s no guarantee that it’s really cured!

and the problem has mysteriously resolved itself

Sounds like a temporary server issue. You could try asking your hosts about it.

Thanks @esmi – clearly it was a temporary issue – but what could be making WordPress show the bar on normal site pages, but not in the admin area?

My first thought was CSS – but then I found that the header was not even in the rendered html page.

It may also have been a (local) Javascript issue, as I think it’s a script that adds the header to the page (?).

… so it may also be plug-in related: I use iThemes Security (previously BetterWP Security) – which includes a “safe jQuery” option that may have temporarily got in the was.

So – still nothing conclusive, but some ideas ready should the issue arise again.

hat could be making WordPress show the bar on normal site pages, but not in the admin area?

It’s really impossible to say now that the issue has vanished. It could have been any of a 101 things.