Use Query strings to pass value from one page to another.And you can access the values from the query string.
Send the required value to the target page(Let it be Default2.aspx) like this
Response.Redirect("Default2.aspx?pid=" +txtData.Text);
You can access the values at the target page(Default2.aspx).
eg:
string projectid = Request.QueryString["pid"];
You can send multiple values throug query string like this using '?' and '&'
Response.Redirect("Default2.aspx?pid=" +txtData.Text + "&fid=" + txtcountry.Text);
and access the values in same way.
string projectid = Request.QueryString["pid"];
string FeatureId= Request.QueryString["fid"];