Hello everyone
Thank you so much for all your responses!
I tried all the suggested methods but they didn’t work. The event banner was pinned but somehow the associated topic was unavailable or had been overridden/overwritten somehow.
Managed to fix it though - pinned a new topic and then unpinned it, which did the trick!
Once again, thank you for all the help and prompt responses - they definitely sparked different thoughts and idea, which led me to the solution 
You all are awesome!