Ok. Here is a scenario :
1. sqlplus hr/hr@T101
2. create dblink - JOPRIM_DBLINK to access JOPRIM db.
3 select sysdate from dual@JOPRIM_DBLINK
4. Result 9/12/2010 4:15:53 PM - all fine !
5. .oraenv JOPRIM. slqplus '/as sysdba'. startup force. - it will bounce JOPRIM database.
6. select sysdate from dual@JOPRIM_DBLINK (I am still in the same session from step1)
7. ORA-02068: following severe error from JOPRIM_DBLINK
ORA-03113: end-of-file on communication channel
8. select sysdate from dual@JOPRIM_DBLINK (do it again)
9 . 9/12/2010 4:47:01 PM all Ok!!!

Note it happens because I am in the same session if I logout and login this won't happen.