Review the original item:
Susan ducked when she saw Professor Hayden walk into Tito's Taco
Palace,
unfortunately, a burrito was not big enough to hide behind, so poor
Susan had to explain why she had missed class again.
You wanted to fix it this way:
Susan ducked when she saw Professor Hayden walk into Tito's Taco
Palace;
Unfortunately, a burrito was not big enough to hide behind, so poor
Susan had to explain why she had missed class again.
A semicolon is an excellent way to fix a comma splice. You must be careful, however, that you don't capitalize the word that follows the semicolon. Unless the word following the semicolon is a proper noun, one that is always capitalized, you should leave it with a lower case letter.
You might want to consult the rules for fixing comma splices and fused sentences.