Thanks, Charles.
How did it break? I was standing at the top of some steps aiming the camera down at someone below and when I half-pressed the shutter to focus, the entire inside part of the lens dropped forward, making a grinding sound. It didn't fall out, just stayed lodged at an angle. I very carefully pushed it back in but it wouldn't autofocus without making the grinding sound and wouldn't manually focus properly either: something inside seemed to be stuck in the wrong position.
I put it in the cupboard for quite a while then, after reading an article and viewing a video on how to fix the nifty fifty, I got it out to have a look. I tried pulling the middle part out a way to see if I could see what was wrong with it, then pushed it firmly back in. I then tried rotating it as if I were manually focusing with it locked in auto-focus, and it didn't make the grinding noise any more! I put it on the camera and tried autofocusing and it wouldn't do it, but it was now fine when I tried manual focusing. The centre of the lens is still loose and feels fragile, but if I hold it carefully, I can get a reasonable shot out of it, though I'm constantly worried that the middle part will drop out again if I tilt it too far forward.
I took it to a local camera shop and they wanted around £130 to fix it - the lens is available on amazon for £84 so I'm still waiting until I've got enough spare money to buy it. In the meantime, I'll make do with the broken one.
While it was in the shop, they found a crack in the middle of the lens that looks like a chip about 2.5 mm wide. They claim that it was damaged when I brought it in, of course. It amazes me how much damage a lens can take and keep on producing reasonably sharp images.
