Audacity not playing back?

Check your soundcard recording level, it may not be high enough. Unless you can see an actual wave after the recording, that's the likely problem.

If you can see a visible wave, then I'll need a think.

Well, I'm answering because I've had this problem too. I fixed one of the codecs and it seemd to work again, but that took me two hours. Try rebooting, sometimes the thing that causes the problem recurs because the programme remembers. At the worst try reinstalling, that might update everything and fix the problem.

Sorry if this not helpful enough, sometimes software goes funny for no clear reason.

edit2 - Okay, its hard for me to fix a fault unless I can tinker. On the off chance it doesn't work any longer, try using Reaper, it's free and much more powerful. It says it's not free, but it is, I've been using it for a long time.

http://www.cockos.com/reaper/download.php

Have you tried Edit>Selection>All & then green arrow? (for start to finish playback)

For other selections, place cursor over the wave form, tick, then hit green arrow....