This used to be so easy. I used to just go onto 'Purchased' and look on 'Not on this Computer' - but now that's gone? I wanted to redownload a song I purchased a long time ago... Any advice?

What version of iTunes are you using and are you using PC or Mac?

I use a Mac and this happened to me on iTunes 11.

I eventually found the track on my Mac which i'd obviously managed to partially delete as it wasn't found when searching within the iTunes app. I found it by using Finder and looking in my Music folder. I then deleted it, logged onto iTunes and Hey Presto! it was then available to download again.

Also worth checking that the Apple ID you're using is the one you used to purchase the track (if you have more than one Apple ID)

Find it on YouTube.
Bound to be several uploads, so pick a good quality one.
Download it as an mp3.

Simples.

What do you see when you search for the track via the iTunes Store?
If it shows a purchased icon then you should be able to download it again.