If the message file is there. Then the root cause of this problem is usually the PATH.
Make sure you correctly set your PATH in your system environment.
Here is the step to check that the problem really is your PATH
C:\>set ORACLE_HOME=<your Oracle Home> (just above bin directory)
C:\>sqlplus “/ as sysdba”
If you can run sqlplus then you know for sure that there is something wrong with your PATH.
Go to your system environment and create ORACLE_HOME. Its value should be directory to your oracle home ( mine is C:\Oracle\product\11.2.0\client_1 , make sure your dont leave a trailing ‘\’ or ‘;’ )
Then edit your PATH and put %ORACLE_HOME$\bin; at the leftmost of your PATH value.
Restart cmd and try sqlplus. It should work now..