Hi  asokandell,

Welcome to the Community,

Since the external keyboard is working fine,most likely you will need to get the system keyboard replaced.

However you can try the trouble shooting steps given under Troubleshooting a Keyboard or Touchpad Built Into the Notebook,in the link below :

support.dell.com/.../document

If the issue persist replace the keyboard.Keyboard replacement is covered by warranty.Is your system within warranty?

If warranty has expired and you want to order the keyboard,Dell part number for the keyboard is  9GT99. Check this link for more information on keyboard : www.parts-people.com/index.php

Hope this helps,

Thank You

thanku very much roshan g

You're Welcome