Solution for ORA-01157

Recently while dropping the database some of my other datafiles gone missing. Because of this when trying to open the database got an error "ORA-01157 - Cannot find lock for the file D:\oracle\Oradata\Oradesigner\Designer" and the database is not at all opening

Solution
1. Login to database as / as sysdba
sqlplus > connect / as sysdba
SQL*Plus: Release 9.2.0.8.0 - Production on Thu Feb 8 14:54:38 2007
Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
Connected to an idle instance.
sqlplus > startup mount;
ORACLE instance started.
Total System Global Area 236404368 bytes
Fixed Size 724624 bytes
Variable Size 201326592 bytes
Database Buffers 33554432 bytes
Redo Buffers 798720 bytes
Database mounted.
sqlplus > alter database datafile 'D:\oracle\Oradata\Oradesigner\Designer' offline drop;
Database altered.
sqlplus> alter database open;
Database opened