Problem Looking Strange For me..,
May be your all controls not set as runat server
here is the code i am providing just done on my PC
protected void Button1_Click(object sender, EventArgs e)
{
TextBox1.Text = String.Empty;
TextBox2.Text = String.Empty;
}
HTML Code
<body>
<form id="form1" runat="server">
<div>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><asp:TextBox ID="TextBox2"
runat="server"></asp:TextBox><asp:Button ID="Button1" runat="server"
Text="Button" onclick="Button1_Click" />
</div>
</form>
</body>
It's working For me....