Internal Server Error Problem while clicking on the Add Missing Products to the index and Re-build the entire index

[kirubanidhi]

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator to inform of the time the error occurred and of anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

 

How to resolve this error. This problem occuring while clicking on the Add Missing Products to the index and Re-build the entire index (Preferences > Search).

 I am using Prestashop version 1.5.6 Not upgrade to latest version. In my front page, I search the product. It getting the error "No results were found for your search "638-5100".

How to resolve my problem, Please help me,