You can do it a bunch of different ways, such as hidden fields above. To do it using ViewState I would do the following.
int total;
int num = Convert.ToInt32(txtScore.Text);
if (ViewState["total"] == null)
{
total = 0;
}
else
{
total = (int)ViewState["total"] + num;
}
ViewState["total"] = total;
Label1.Text = total.ToString();