There's no '+' sign to bring it back, and turning on "editing" doesn't make a difference either.
Re: Student clicked the '-' and the Message block disappeared
Are you using Moodle 2.0? Has it been docked at the left by accident?
Re: Student clicked the '-' and the Message block disappeared
I'm using 1.9, so I don't think that's it.
Apologies, I should have specified in my initial post.
Re: Student clicked the '-' and the Message block disappeared
Which 1.9? Are you using any pinned (sticky) blocks? If so, you might be running into MDL-11131 (fixed in 1.9.8).
Re: Student clicked the '-' and the Message block disappeared
It turns out that every single block in the right-hand column acts this way: when it's clicked, it just vanishes forever.
The left-hand column blocks work as they ought to.
I've also discovered that switching to a different browser brings up the site with all blocks visible, which suggests a cache problem - but clearing the cache does not fix it.
Very perplexed!
Re: Student clicked the '-' and the Message block disappeared
Maybe a theme problem. Which theme are you using?
Re: Student clicked the '-' and the Message block disappeared
I agree that theme is a good starting point, also do you have drag and drop enabled? This is often the cause of disappearing blocks in 1.9.x.
Re: Student clicked the '-' and the Message block disappeared
I hired someone to look into this - turns out it was some kind of CSS error within the theme. Good call, Joseph.