connect to db oracle using plsql

hello to all, and thanks of your aid, only, that I have tried to write a function with the cursore having a select * from schema.tabella_at_db_link, that if db the correspondent to the db_link is not connected the function only blocks and from these errors, without the possibility to manage them through exception.Come making then. Thanks still hello claudia

errors:

ORA-04052 ERRORE VERIFICATOSI DURANTE LA RICERCA DI OGGETTO REMOTO NOME_SCHEMA.TABLE_at_DB_LINK ORA-00604 ERRORE RISCONTRATO IN SQL RICORSIVO LIVELLO 1 ORA-12535 TNS OPERAZIONE FUORI TEMPO Received on Sun Jun 02 2002 - 11:11:41 CDT