Hi, clunkster. Anything is possible, of course, but from what I know:
It is unlikely that your forum has been actually hacked, as it requires hacking ForumUp rather than a single forum.
The most likely way for someone to "hack" your forum is to get into the Admin's panel -- using an existing Admin account -- and make a mess.
To be safe rather than sorry:
Make sure that you can get in there. Check that everything is okay, and check that no new-ish members have appointed themselves Admin all of a sudden.
And last but not least, change your password. Even if everything seems okay. If you are more than one Admin, then all of you need to this.
But they could have meant you who clicked the link have been "hacked". So check your computer with every scanner you have; virus, spyware, adware etc. And be observant how it "behaves" in general.
FoxLeonard