As blue_arc said, you use user control from page codebehind same way as if it was standard ASP.NET web control. Assuming you have in your aspx file:
<uc:userCtrl runat="server" id="myCtrl"/>
in codebehind do:
string password = myCtrl.pPass;
Is it clear? :) You should not try to access textboxes directly, as it goes against encapsulation principle (hence I downvoted guy above me :P).
"Throughout human history, we have been dependent on machines to survive. Fate, it seems, is not without a sense of irony. " - Morpheus