Hi OP
It's possible you may have hidden the thread. It's easy to sort if that's the case.
First go to the main Talk page and click on 'Customise'.
Then, if you scroll all the way down to the bottom of the page, there is a blue box which says 'Ignore topics in Active Conversations'.
In this box will be a small link: www.mumsnet.com/Talk/topic-choice which takes you to a page that will show you all your active conversations, but also anything that you've excluded, and this will allow you to include that thread again.
We hope that clarifies things for you but please do ping us a line if you have any further problems.