I had this problem after queuing multiple episodes. After I had left the page, I could not get back to my list of queued episodes from Dashboard. So after digging the list of episodes out of my browser history, I copied the URL, tested in in a couple of new tabs, and then saved it in a notepad document juuust in case.
So as @Evan says, copy the link(s) you need, and maybe save them somewhere. You can actually have a buffer on Webtoon in this manner, but you absolutely have to publish them in the order you uploaded them so be sure everything is correct about your buffer episodes. As far as I know, we can't go back and insert another episode, but we can edit existing episodes.