iPhone not recognized by iTunes after not ejecting properly
The built in help (on Mac at least) doesn't have the good troubleshooting articles that http://support.apple.com does.

I would start with running a Device Sync Test in iTunes and if that won't even run, you can fall back to the Device Connectivity Test.

That should pinpoint the source of the issue if it's not software to:


OS issue
USB hardware failure (on the computer, cable or device itself)
Hardware failure of the device (get it repaired / restored by taking it to another computer)