iMessage no longer working on Mac after AppleID change
The vast majority of the time when iMessage no longer delivers to a Mac (or iOS device) is due to the time settings where the clock has drifted.

I would work through this checklist as you rule out any account related issues (especially the part about appleid.apple.com to verify the account isn't locked):


http://support.apple.com/kb/TS3970


It also covers a big reason why messages fail to be delivered even if you can sign in - firewall settings and networking issues between the Mac and the APNS servers that alert your computer a message is coming and help deliver the message itself.