Textbox value is loosing

The problem is when you post the form then the state changed by Javascript or Jquery of controls would'nt get submitted so you have take care of that. so if you are using jquery modal pupup embed it to the form and it'll get submitted with the form with the text box value..
Try the code like this:

$("#divSaveAs").dialog({bgiframe:false,
                            autoOpen:false,
                            title:"Save As",
                            modal:true});
    $("#divSaveAs").parent().appendTo($("form:first"));

Check the page_load event of control see if there is any code that is clearing the data.. try using the viewstate for the TextBox control. These are other basic checks you can try..

Hope this helps you..

Thanks ;)