That's happened to me before when Media Player hasn't shut down properly. The plug in gets disabled.

Go to Tools > Options and click on the Plug In tab. You'll see a list of categories. Click Background and you should see "Last.fm Plugin in the box to the right. Check the ticky box next to it and it should start scrobbling again. I hope that works!

How do you play other people's favorite songs? *is curious*