Try to connect by "native" sql/plus (if it is installed) first to isolate the problem. If the native connection doesn't work either then you should check your Oracle (client) installation, your tnsnames.ora and see whether your client version is still supported for the version of the Oracle database you want to connect to.
If the native connection works you should check PLSD settings: Preferences>Oracle>Connection ("oracle home", "oci library").
