Hi,
Not seeing your code I can only guess, but this is a very common mistake that people who are new to ASP.NET development make. You need to understand the lifecycle of page events. The page load event fires before any button click events on postback so if you are populating the controls with initial values in page load then the values entered by the user are lost before the button click event fires.
So the solution is to wrap any logic you have that sets the initial values like this:
if(!Page.IsPostBack) { do your logic to set initial values}
Hope that helps,
Joe