@ OsVestaDyce - If a thread does not have recent activity, it can slip towards the end of the forums, and if it reaches the end of page 50 it falls off the forum, (more commonly known as being eating by the Page 51 Monster.) Once a thread has fallen off the forums, there is no way to retrieve it.
We always encourage thread authors to keep an offline copy of their thread, especially if it's lengthy or contains important information that you don't wish to lose. If you have not done this, I'm afraid that you will have to restart it from scratch. (Sometimes you can get lucky and find a cached copy on a certain popular web search engine.)
Thread author's have a 'bump' button at the top of their thread, so that if the thread does start to make its way towards page 50, it can be 'bumped' back to page 1 and ensure the thread's survival for bit longer.