Check your PopUpTitle CSS, sometimes the AJAX fired and worked, but the proper css was not available, to alter. In other words, the modelopopupextender just makes a change to the css, which makes it pop up, and then changes the css back to the original parameters to hide the popup. So change your css to show the popup, and change the css back to hide the popup, before implementing the popup extender. If the popup extender still doesn't work after that, then your missing a extender property, which should trigger an error. I didn't see the close button target id. Test your css class for show and hide by altering the css This should be show or hide protected void btnIntroductionText_Click(object sender, EventArgs e) { MPEProphetAndLoss1.Show(); // Show or Hide } }