unable to connect to imessage server

Read here:

Using iMessage

http://support.apple.com/kb/HT3529

Troubleshooting iMessage activation:

http://support.apple.com/en-us/HT201422

You could try disabling iMessage by going to your iPhone > Settings > Messages disable iMessage, then force close the Messages app by double pressing Home button and swipe away Messages app.

Restart your your iPhone then go to Settings > Messages and enable iMessage.

Test to see if the issue has been resolved.