Blank Messages Screen
Even though I have added my iCloud account, messages screen is gray and couldn't make it work. I am also unable to click preferences on messages, the menu option is grayed out.
iMac, OS X Yosemite (10.10.5)
In the Finder > Go Menu hold the ALT key down and then select the Library that appears.
Navigate to the Preferences Folder.
Find com.apple.ichat.plist and drag it to the Trash.
Now restart the App again.
If this does not work open the Keychain Access app in Applications/Utilities and then using the KeyChain Access menu > First Aid reset the Keychain.
Second one solved it.
Blank Messages Screen