Not all parts of your code return a value..
Try this code:
public ActionResult Page()
{
//LINQ x expressions
//LINQ y expressions
if(Request.QueryString["type"] == "x")
{
return View(linqExpX.ToList());
}
else if(Request.QueryString["type"] == "y")
{
return View(linqExpY.ToList());
}
return someDefaultView;
}