Try this....:)
<asp:button id="btnBack" runat="server" text="Back" xmlns:asp="#unknown">
OnClientClick="JavaScript: window.history.back(1); return false;"></asp:button>
or
private void btnBack_Click(object sender, System.EventArgs e)
{
string prevPage = Request.UrlReferrer.ToString();
Response.Redirect(prevPage);
}