When the TextMode property of a TextBox control is set to Password, the value you assign to the Text property (either declaratively or programmatically) isn't actually displayed at runtime, not even in masked mode (with the * chars). This is of course done for security reasons, because even if the password would be shown in masked mode, the actual clear-text password could still be retrieved from the page's source HTML code.
However, there are times when you actually want to pre-fill the password field, for example in a "My Profile" page where the user can change his personal data, including his account's password. If you don't protect very sensitive information, you may decide that showing the password in masked mode is a good compromise to increase the user's experience. (The option of not showing the password at all and only update the password if the user types in something is not as intuitive for the user...). It would have been nice to have a direct option to show the password in masked mode but, as said, nothing appears if you set the TextBox's Text property. The trick is very simple though: you just have to manually add a "value" attribute to the HTML generated by the server-side textbox control, where the value is the password. You do this by accesing the control's Attributes collection, as shown below: