I have a Fast Track Pro, so I'm not sure how much of this will apply to you, but every now and again it gets ornery and does about the same thing.
So far, following one or more of these steps has been able to solve the problem for me each time.
1. Confirm that you have installed the latest M-Audio drivers, and that all of the M-Audio processes are running. Confirm that you can make modifications to the preamp configuration (enable/disable S/PDIF, second channel or output group, etc.) to make sure the drivers are working.
2. Power down, then power up the interface, and then restart the audio application(s).
(Speaking of, if you have anything that's actively running audio - even silence - through the interface other than your recording instance of GarageBand, you should probably exit it just to be sure that nothing's locking the drivers.)
3. Confirm that either a) all insert-jack gear is working properly and is receiving and sending signal, or b) there is nothing plugged into the insert jack.
4. Reduce bit depth on both interface and software.
5. Disable software monitoring, and use the hardware loopback.
6. (I hate to mention this one, but I suppose we've all been here once or twice) Confirm that either a) you are using a dynamic mic, or b) that you have enabled phantom power for the condenser circuits. Also ensure that your gain knob is set fairly high - I've found that most condensers warm up nicely at around 75-80% gain on this preamp.
7. Reboot the computer. Some days, it just wants a little attention.
If none of the above work, attempt to record a line-level signal such as a guitar, or even an mp3 player. Confirm that your interface does receive signals in the first place. Try recording with another program, particularly something basic that only does recording. Try using your interface on another computer; even - gasp! - a Windows box. Do make sure to install the drivers before connecting the device or it won't work properly. If you use Windows, the built-in Sound Recorder works fine for testing.
And if everything fails, call M-Audio and demand a warranty replacement.
