Apart from all the suggestions above, you should ensure that the scriptmanager has the script references pointing to the right files. There are possibilities that you can get this path to be mistated in your file. I am talking about the below section.
<asp:ScriptManager ID="scriptmanager1" runat="server">
<asp:ServiceReference Path="path to the asmx here" />
Open the website thru Internet Explorer
and look for network Errors. Both the browsers, give you sufficient information to figure out failing requests.
One way to figure out the missing files is to use developer tools in almost every modern browser or use Fiddler and check the broken requests.
For developer tools in IE - Press F12, and go to Network tab, and press start capturing.
Or in chrome, use the Network tab to figure out all 404 requests.
Hope this helps.