If ever you've tried setting value in code-behind to a textbox with TextMode property set to "Password", you would notice it doesn't display the masked text on the page. This is simply the way it is for security purposes, but there is a workaround so you can show the user there is actually text in the textbox.
Here is the snippet:
txt1.Text = "sample_password";
if (txt1.TextMode == TextBoxMode.Password)