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
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
ORA-01157: cannot identify/lock data file 20 - see DBWR trace file
ORA-01110: data file 20:
In order to resolve the above issue, perform the following and restart the database.