You are using a relative path to the page. You should be using a full path.
When referencing pages/images/scripts within a ASPX page you should include the full path from the root directory.
Lets say you have the following structure;
Default.aspx in root directory
SiteMaster.Master in folder named Master
Signin.aspx in folder named Singin
Contact.aspx in folder named Contact
About.aspx in root folder
The full path to each of these files would be;
~/Default.aspx
~/Master/Sitemaster.Master
~/Singin/Singin.aspx
~/Contact/Contact.aspx
~/About.aspx
The "~" character indicates the root of you website
You really shouldn't use relative paths
Refer:
ASP.NET Web Project Paths[
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