Question

The team attacked but couldn’t score a goal. Then, through a/an ordinary piece of good fortune, the other team accidentally kicked the ball into the wrong goal.

Answer 1

I’m assuming your asking if your supposed to use “a” or “an”.
Since where your putting a/an is followed by the word ordinary, which starts with the vowel “o” you would use the word “an”. Anytime the following word starts with a vowel you use an.