For this you can use UpdateProgress.
Use UpdateProgress like this-
<body>
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server" />
<asp:UpdateProgress runat="server" id="PageUpdateProgress">
<ProgressTemplate>
Loading...
</ProgressTemplate>
</asp:UpdateProgress>
<asp:UpdatePanel runat="server" id="Panel">
<ContentTemplate>
<asp:Button runat="server" id="UpdateButton" onclick="UpdateButton_Click" text="Update" />
</ContentTemplate>
</asp:UpdatePanel>
</form>
</body>
The following method should be added to your CodeBehind file:
protected void UpdateButton_Click(object sender, EventArgs e)
{
System.Threading.Thread.Sleep(5000);
}
Hope this will help you.