Suddenly getting ORA-12514 error Oracle 11gr2

I installed Oracle 11g r2 fine, had 2 databases (se3 and mydb, both have db_domain .orcl i.e. mydb.orcl and se3.orcl) running on it fine till yesterday. but suddenly im getting ORA-12514 error (TNS:listener doesn't know of service requested in connect descriptor)..

My tnsnames.ora file:

# tnsnames.ora Network Configuration File: G:\oracledb\product\11.2.0\dbhome_1\network\admin\tnsnames.ora
# Generated by Oracle configuration tools.

LISTENER_MYDB =
  (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))


ORACLR_CONNECTION_DATA =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
    )
    (CONNECT_DATA =
      (SID = CLRExtProc)
      (PRESENTATION = RO)
    )
  )

MYDB =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = mydb.orcl)
    )
  )

SE3 =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = se3.orcl)
    )
  )

LISTENER_SE3 =
  (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))

LISTENER.ORA FILE:
# listener.ora Network Configuration File: G:\oracledb\product\11.2.0\dbhome_1\network\admin\listener.ora
# Generated by Oracle configuration tools.

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = CLRExtProc)
      (ORACLE_HOME = G:\oracledb\product\11.2.0\dbhome_1)
      (PROGRAM = extproc)
      (ENVS = "EXTPROC_DLLS=ONLY:G:\oracledb\product\11.2.0\dbhome_1\bin\oraclr11.dll")
    )
  )

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
      (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    )
  )

ADR_BASE_LISTENER = G:\oracledb

I am able to "tnsping mydb/se3" fine.. Tried to change tnsnames.ora and listener.ora files too (now restored) but cannot connect to either one of them through sqlplus or sqldeveloper/visual studio.. plz help me.. thanks!

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