add script code in your page:
<script type="text/javascript">
function Showalert() {
alert('Call JavaScript function from codebehind');
}
</script>
here is code for display the alert message... It is similar to msgbox.
<pre>
<div>
<asp:Button ID="btnSubmit" runat="server" Text="Show alert" onclick="btnSubmit_Click"/>
<asp:Button ID="btnClick" runat="server" Text="Call JavaScript Function" onclick="btnClick_Click" />
</div>
I hope this is help for You.