Root element is missing

Hi this is odd way but try it once

1. Read the file content into a string
2. print the string and check whether you are getting proper XML or not
3. you can use XMLDocument.LoadXML(xmlstring)

I try with your code and same XML without adding any XML declaration it works for me

XmlDocument doc = new XmlDocument();
        doc.Load(@"H:\WorkSpace\C#\TestDemos\TestDemos\XMLFile1.xml");
        XmlNodeList nodes = doc.GetElementsByTagName("Product");
        XmlNode node = null;
        foreach (XmlNode n in nodes)
        {
            Console.WriteLine("HI");
        }

As stated by Phil in below answer please set the xmlStream position to zero if it is not zero.

if (xmlStream.Position > 0)
{
    xmlStream.Position = 0;
}
XDocument xDoc = XDocument.Load(xmlStream);