This has been a problem for me for a long time. Usually I have to turn off Wi-Fi on my Macbook Air and my iPhone and then turn them both on.
If that doesn't work a soft reset of my iPhone sometimes fixes it (hold the power button and the home button for several seconds until the background turns white).
If they still won't connect, a reboot of the Macbook Air is next.
If that still doesn't work, turn Sync with this iPhone over Wi-Fi off, sync, turn Sync with this iPhone over Wi-Fi on, and sync one more time.
If that still doesn't work, I power both the Macbook Air and the iPhone off and then start the Macbook Air first.
Usually somewhere along the above steps, my iPhone reappears in iTunes over Wi-Fi for a few days until the whole annoying process needs to be done again.
Good luck!