Javascript - Button Is Not Fired Normally And Fired When UseSubmitBehavior="false" But Textbox Values Are Coming Null?

I am using ASP button its working properly at client side but on server side its not firing.

<div style="text-align: center">
<div>
<div id="UserName">
<b>LoginName</b>
<asp:TextBox ID="txt_LoginName" runat="server"></asp:TextBox>
</div>
</div>
<div>
<div id="Password">
<b>Password </b>
<asp:TextBox ID="txt_Password" TextMode="Password" runat="server"></asp:TextBox>
</div>
</div>
<div style="text-align: right; width: 210px;">
<asp:Button ID="btn_SignIn" OnClientClick="LogInUser()" UseSubmitBehavior="false" runat="server" Text="Login`enter code here`" />
<asp:Label ID="lbl_InValidError" runat="server"></asp:Label>
</div>
</div>

Client Side Code:

function LogInUser()
{
var LoginName = document.getElementById("<%=txt_LoginName.ClientId %>").value;
var Password = document.getElementById("<%=txt_Password.ClientId%>").value;

[Code]....