Disconnection Forced (ORA-01092) Error

When I tried to connect to Oracle It gave an error message "ORA-01092: ORACLE instance terminated. Disconnection forced" then tried to such solution

SQL> conn /as sysdba
Connected to an idle instance.
SQL>shutdown immediate
SQL> startup nomount
ORACLE instance started.
Total System Global Area 1577437669 bytes
Fixed Size 798035 bytes
Variable Size 172536676 bytes
Database Buffers 24164814 bytes
Redo Buffers 243145 bytes
Database mounted.
SQL>Alter Database recover;
SQL>Alter Databse open

It worked.