We have two solution for this.
1. Connect locally that is without service name.
set ORACLE_SID=<your sid>
set ORACLE_HOME=...
sqlplus "/ as sysdba"
or sqlplus "sys as sysdba"
2. Define an entry in the tnsnames.ora with (SERVER=DEDICATED) in the CONNECT_DATA section and connect using this name.
1. Connect locally that is without service name.
set ORACLE_SID=<your sid>
set ORACLE_HOME=...
sqlplus "/ as sysdba"
or sqlplus "sys as sysdba"
2. Define an entry in the tnsnames.ora with (SERVER=DEDICATED) in the CONNECT_DATA section and connect using this name.