var conn = new SqlConnection("<connection string>");
conn.Open();

var cmd = new SqlCommand("SELECT CNIC FROM ACCOUNT WHERE CNIC = '" + textBox1.Text + "'", conn);
var reader = cmd.ExecuteReader();

while (reader.Read())
{
    if (reader.HasRows)
        {
            // ACCOUNT ALREADY EXIST
        }
        else
        {
            // CREATE ACCOUNT
        }
}

conn.Close();