Message Box with Yes No button in ASP.Net

I need a message box in asp.net web application which has yes and no button.

I have asp.net webapplication page to save certain values on clicking submit.

But if the user enters some wrong mesage , an alert message box should be displayed.

 when yes is clicked the condition should satisfy and if  no the user can't save the value.

How to do it? In most cases alert message is displayed but the values get saved for both ok or cancel  . How to solve.