404 not found!
What happened?

404 - Page Not Found, so either the thread has been deleted or you have been the latest victim of the Alacrity daemon.
But my thread isnt deleted? Where is my thread?

If you have arrived here after clicking a thread then please try this: add sys. to the beginning of the broken URL.

For instance:
    8ch.net/[board]/res/1111.html
would become
    sys.8ch.net/[board]/res/1111.html

Now, you will be able to view the thread. If you choose to post on it, the thread will be fixed. This is a temporary workaround until the 8chan Administrator finds a permanent solution for this bug.
Here are the same instructions in an image.
But I dont have a mod.php account, how can I login?

If you do not have a mod.php account, there is a public account that anybody can use. The credentials are:

Username: Anyone
Password: 0