Consider a scenario where you have accidentally deleted a DATA-FILE in oracle when the instance is running. It will throw error when you restart the DB as follows.
1. Log-in to oracle DB server
$ sqlplus system/manager as sysdba
SQL> startup
ORACLE instance started.
Total System Global Area 1174405120 bytes
Fixed Size 2020288 bytes
Variable Size 285215808 bytes
Database Buffers 872415232 bytes
Redo Buffers 14753792 bytes
Database mounted.
SQL> startup
ORACLE instance started.
Total System Global Area 1174405120 bytes
Fixed Size 2020288 bytes
Variable Size 285215808 bytes
Database Buffers 872415232 bytes
Redo Buffers 14753792 bytes
Database mounted.