SQL> shutdown immediate;

SQL> startup mount;

ORA-24324 service handle not initialized
ORA-01041: internal error, hostdef extension name not exists

SQL> shutdown immediate;

SQL> startup nomount;

SQL> startup nomount;

SQL> conn sys/sys as sysdba;

SQL> startup;

Total System Global Area  535662592 bytes
Fixed Size                  1334380 bytes
Variable Size             234881940 bytes
Database Buffers          293601280 bytes
Redo Buffers                5844992 bytes