iMessage not working on mac
it won't even let me open the app on my Mac and I'm logged into my iCloud
Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?
Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.
Still no workie?
To find out if it's system wide or user specific, try this...
Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.
Does it work in the new account?