You can do this with the help of "Enabled" property. You need to set Enabled=false to make text box disabled.
By default text box is enabled.
Here is example for this.
In this example we have one text box and we make that textbox disabled.
You can set this property from aspx side or from Coding side.
<asp:TextBox runat="server" ID="txtProductTitle" Text="Keyboard" Enabled="false" ></asp:TextBox>
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