Currently Being ModeratedSep 20, 2013 12:30 PM (in response to geenarenaye)
Do you have Parental Controls turned On that is restricting access to Messages ?
Parental Controls is in System Preferences > Users and Groups the account you are logged in to your Mac with has to be a NON Admin one to allow Parental Controls to be Activated (it then gets listed as Managed).
Parental Controls can limit the access to certain Apps and block who you can communicate with in Messages and Mail.
Aside from this there can be occasions when the Accounts in Messages (or iChat) don not login but you have no access to the Preferences.
In this case use a Finder window > GO Menu > Go to Folder and Copy and Paste into the Dialogue box the following:-
Once in the Preferences find the com.apple.ichat.plist (Messages still uses this one)
Drag it to the Trash then Restart Messages.
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously