If your thread has been merged that means you have duplicated an already existing thread. You can still click the link to your thread, but you will skip all the posts that were made in the original thread.
In the past, we used to lock duplicate threads and provide a link to the original.
Then Tino introduced merging, but we had to put it on a list for him to check every time he came online because only he could merge.
Now he has expanded it to all moderators and admin so you will see it happen more often now.
It is a common feature used on many other forum sites. If you do not want your threads being merged then please do a search to see if someone else has started a similar or identical topic before posting.