Problem when PI/XI starts

Hi...

When my PI instance is starting, I receive this syslogs messages...

Process:

Running but not connected to Message Server, J2ee status info unavailable

Syslog:

Database: Database error 1034 at CON

SAP-Basis System: Initialization DB-Connect Failed, Return Code 000256

Database: SQL error 1034 occurred (possible network problems?). Work process is stopped

Database: > ORA-01034: ORACLE not available

ORA-27101: shared memory

What can I do?!

Thanx