Because you are concatenating your sql statement together you are probably missing a single quote or your data has a quote in it which breaks the sql statement. It would be very easy to debug your code and see what the UPDATE statement actually looks like.
To fix this you should use parameterized queries. For example:
String sql = "UPDATE table SET field = @value1";
using (SqlCommand cmd = new SqlCommand(sql, sqlCon){
cmd.Parameters.AddWithValue("@value1", row.Cells[0].Value.ToString());
...
}