Here is the item again:
Smiling with satisfaction
(A) Michelle typed
the last sentence of her research paper. Then a bolt of lightning crashed
nearby,
(B) extinguishing
the electricity (C)
and making poor Michelle wish that she had invested in a surge protector.
You correctly realized that a comma was necessary after satisfaction. Smiling with satisfaction is a participle phrase. When a phrase of this type introduces a main clause, you need a comma to connect the two parts.
To fix this problem, you chose to do this:
Smiling with satisfaction,
Michelle typed the last sentence of her research paper. Then a bolt
of lightning crashed nearby, extinguishing
the electricity and making poor Michelle wish that she had invested in a
surge protector.
You have correctly employed Punctuation Rule 4:
Participle Phrase + , + Main Clause.
Well done!