Have you downloaded the latest driver for the Duet from Apogee's website?
What version of OSX are you running? 10.5.8?
In terms of my previous post... were you able to try all the different steps... including using different microphone(s)/cables?
The Maestro software will allow you to set all sorts of levels and options - just wanted to make sure you're familiar with that.
Open Garageband and create a "Real Instrument track". When you record the mic's input do you get a good signal there?
It is possible for any audio interface to break... though I've not heard of such a dramatic case with the Duet...
You can check in:
Applications/Utilites/Audio MIDI Setup that the Duet is recognized there...
Also check in System Preferences > Sound [Input] that the Duet is listed there...
... but really you can set Logic's sound settings in Preferences > Audio independently to the OS sound settings.
Let me know how you get on.