使用ajax分配文本框值 - 页面仍然刷新，分配的值已消失

I am trying to fill a part of my web page by using AJAX instead of C# since the webpage should not be refreshed for this action. All I need to do is to run a SELECT query and get the current client from SQL DB and fill the related form without refreshing the page. I managed to get the related client and to change the related textbox, however, the page is still refreshing and all the textboxes becomes null again!I am trying to fill a part of my web page by us