There are two ways to find the notes that correspond to a now cancelled or deleted event:
1) Use the "Show Cancelled Meeting" button
There is the option to show cancelled meeting on your calendar. This will allow you to re-visit notes from meetings that have been cancelled (or hide them from view 🙈)
The button is located in the meetings tab at the top of the calendar. Click on the three dots (...) to show the drop down menu.
2) Use the Search Function
You can access those old notes by using the Search Function in the navigation bar and searching for the name of the meeting, or any keywords.
If you still can't find what you're looking for, contact firstname.lastname@example.org and/or U2 :)