Thanks to all I found a solution for my problem, I just to close the button in update pannel and use a scriptmansger over it.
<script type="text/javascript">
function Showalert() {
$('#innerMsgDiv').css({ "display": "block" });
$('#innerMsgDiv').addClass('animated flipInY');
}
</script>
<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Button class="pull-right btn btn-info" ID="btnSendM" runat="server" Text="Send Message" CausesValidation="False" />
</ContentTemplate>
</asp:UpdatePanel>
Protected Sub btnSendM_Click(sender As Object, e As EventArgs) Handles btnSendM.Click
If txtName.Text = "Enter Name" Or txtName.Text = "" Or _
txtEmail.Text = "Enter Email" Or txtEmail.Text = "" Or _
txtPhone.Text = "Enter Phone Number" Or txtPhone.Text = "" Or _
txtMessage.Text = "Enter Your Message." Or txtMessage.Text = "" Then
msgDisplay()
Else
contactClass.sendMessage(txtName.Text, txtEmail.Text, txtPhone.Text, txtMessage.Text)
txtName.Text = "Enter Name"
txtEmail.Text = "Enter Email"
txtPhone.Text = "Enter Phone Number"
txtMessage.Text = "Enter Your Message."
End If
End Sub
Private Sub msgDisplay()
ScriptManager.RegisterStartupScript(Me, [GetType](), "displayalertmessage", "Showalert();", True)
End Sub
End Class