1. Go to Active. Scroll down to the bottom of the list and click on Manage hidden threads. You'll get a little dialogue box that shows you which threads you've hidden and gives you the option to unhide them.
2. Go to Customise. Scroll down to 'Ignore Topics in Active Conversations' and click through on the 'here' link. This will take you to a new page. Scroll down to the bottom of that and you'll see a list of Hidden Threads. Again, you should be able to select the threads you'd like to unhide.