Hi @marrymeadam - thanks for asking this. We'll try to talk you through it!
First of all, which version of the site are you using? And was it a thread you posted on or one that you started?
If you're using the desktop site, you should be able to see a line of options underneath the big blue Talk menu - including Active, I'm On, I'm Watching and I Started.
On the mobile site you can either click on the three horizontal lines in the top left of the page, choose Talk and then tap on I Started, or you can go to the blue bar at the foot of any page, tap on I'm On and then select I Started from the menu that pops up.
The iPhone app has a list of threads I'm On (well, threads You're On...) at the foot of any page in Talk.
Hope that helps but please get back in touch if you need any further help. 
And if you still can't find the thread in question, feel free to mail us at [email protected] and we'll do our best to find it for you.