Suddenly Create User Username textbox stops working

 I have a create user wizard set up to allow users to create accounts.
Yesterday it was working, today when I go to load the page I get this error:
Compiler Error Message: BC30456: 'UserName_TextChanged' is not a member of 'ASP.createaccount_aspx'.
which highlights this line of code as being the problem:
<asp:TextBox ID="UserName" runat="server" ontextchanged="UserName_TextChanged"></asp:TextBox>
As far as I know that code is part of the create user wizard code -- I have no idea what is suddenly the problem ??

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