There are many reasons why an account may have been restricted from accessing the game chat and/or private messages. However, in each specific case, these restrictions are applied due to violations of the User Agreement.
Decisions to restrict account functionality are final and cannot be reversed.
But I didn’t do anything! The chat was disabled for no reason!
There are a variety of reasons why your account may have been restricted, however, in every case it will ultimately be due to a breach of the User Agreement.