ORA-03113: end-of-file on communication channel

hi am haing this error when starting my datbase in 12c in solaris

SQL> connect sys /as sysdba
Enter password:
Connected to an idle instance.
SQL> startup
ORACLE instance started.

Total System Global Area 1073741824 bytes
Fixed Size                  3011888 bytes
Variable Size             624954064 bytes
Database Buffers          440401920 bytes
Redo Buffers                5373952 bytes
Database mounted.
ORA-03113: end-of-file on communication channel
Process ID: 1444
Session ID: 1 Serial number: 15972


SQL> alter database open;
ERROR:
ORA-03114: not connected to ORACLE

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