Try this,
<script type="text/javascript">
function findme()
{
var myhiddenvalue=document.getElementById('<%=hid1.ClientID %>').value;
alert(myhiddenvalue);
}
</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Finding Hidden Field Value</title>
</head>
<body onload="findme()">
<form id="form1" runat="server">
<div>
<asp:HiddenField ID="hid1" Value="I am Hidden" runat="server" />
</div>
</form>
</body>
</html>
The most important thing is
var myhiddenvalue=document.getElementById('<%=hid1.ClientID %>').value;
as <asp:HiddenField > is a server control.
Thanks.