Cause : This error is, almost appropriately, often the very first one you get on trying to establish a connection to your database. Simply put, it means that you have named the database you wish to be connected to and Oracle doesn’t know who the heck you’re talking about. This error is often caused by typos, maybe in your connection string, but possibly in your tnsnames.ora file. It is also possible that the tnsnames.ora file is not accessible or does not even exist. If it does, ensure that it contains the service name you are using; also go over it with a fine toothcomb to make sure it doesn’t have any unpaired parentheses or such.
Action : Cross Check tns entry, Check firwall access to connecting server