I had finally got my bike the way I liked it, and then something like this had to happen!

I was cycling along when I suddenly felt a jolt and the chain went round far too easily. I stopped the bike to have a look.

It turns out that the final cog (ie. smallest cog) on my rear cassette was turning instead of the wheel.

I decided to change my bike onto the next cog up to see if I could cycle home on that.

At first everything seemed fine. I lifted the back of my bike up and cranked the pedals. The back wheel spun round like there wasn't a problem.

Then, I got on my bike. The very first time I (barely) pedalled, the second cog started spinning like my other one. So now my smallest and second smallest cogs were spinning instead of my wheel.

I've only had this cassette about a month. How can I fix this problem?