I am going to assume you bookmarked the conventional way. Via your browser. Firefox, explorer, ect....
If the forum was ungraded at some point the browser bookmarks wont point to the same location....
This is the hotlink for
THIS thread......the number will likely change unless the person upgrading the forum insures these hotlinks are not changed.
That being said.....these links wont work any longer.
If you go to the bookmark and click edit.....under name you will see what the thread was called when you bookmarked it. You should be able to search for the title and find the original thread.
You could export all your bookmarks and dump them in excel to create a rule so excel does all the hard work for you...finding the name and searching for the name.
Even with that you still need to bookmark all bookmarks again.
As others have said you could bookmark or "watch" the threads as you find one interesting which will eliminate using your browser.