First count the number of rows present in the table
ds.Tables[0].Rows.Count
And then use loop
for(int i=0;i<ds.tables[0].rows.count;i++)>
{
if (TextBox1.Text == ds.Tables[0].Rows[i]["email"].ToString())
{
Label1.Text = ds.Tables[0].Rows[i]["password"].ToString();
}
}