I'm no expert on HPUX. But if this ever worked then you should find out what was changed and if neccessary pull back you listener file from tape. Check the TNSNAMES.ORA, SQLNET.ORA and LISTENER.ORA. and make sure that everything is ok. You might want to compare the entries to another HPUX box.
Bounce the listener, then if neccessary the box. Also make sure that the user running the database has rights to all of its files. i.e. $ORACLE_HOME/oradata/sid and $ORACLE_HOME/network/admin.