After installing Oracle in my system, I can not edit txt files in the command prompt.
Assuming it has not be deleted (unlikely) your path may have been amended not to include your windows system32 directory which is where edit.com should be found
So %windir%\system32\edit.com should start it regardless of the path settings.
You can check/change your path on the Environment Variables section which is found on the Advanced tab of your System Properties. (ie Right-Click on My-Computer and select Properties)