Here is the corrected sentence:
Yesterday, Professor [Name] said I will collect your papers tomorrow; however, he never did.
The semicolon is used to connect two independent clauses that are closely related in meaning.
Here's why it works:
* "Yesterday, Professor [Name] said I will collect your papers tomorrow" is a complete thought (independent clause).
* "however, he never did" is also a complete thought (independent clause) that adds contrasting information to the first clause.
Let me know if you have any other questions!