iMessage is temporarily down: http://apple.com/support/systemstatus/
Likely either you or the computer did something to interrupt the update process. Your only option now is to restore. See http://support.apple.com/kb/HT1808.
In the future, update through Wi-Fi to prevent potential problems caused in iTunes.
Your phone is stuck in recovery mode, you are going to have to hold down the home button while you plug it in to itunes and restore through recovery mode.