Based on your own comment "im able to set ORACLE_SID but cannot connect via sqlplus ... ORA-01034 - Oracle not available" it's likely that your database instances are down. It is theoretically possible to have the underlying Windows service OracleServiceMYDB running and corresponding oracle.exe in processes, but the database instance being down. Your best bet is to look on the server for the trace directory containing alert_mydb.log and review the last entries.
If the database is simply not running and alert log shows no serious errors, start it like this:
set ORACLE_SID=MYDB
set ORACLE_HOME=D:\my\oracle\home
%ORACLE_HOME%\bin\sqlplus "/ as sysdba"
ORACLE not available.
SQL> STARTUP
SQL> EXIT
lsnrctl services
The last command should show MYDB is registered. Proceed with ORACLE_SID=SE3