Just let your readers know! It's honestly not a huge deal to miss a single update.
I think if you're missing update after update after update then it's time to change your update schedule to something you can manage. But if you just miss something every once in a while when something big happens, you can just say "sorry for the lack of update guys! Had some personal stuff come up, but next week the update should be on time like normal!"
Here on Tapas, I'd post a "heads up, I'm going to have to skip this week" on my wall, and just make a note of "sorry for the missed update last week, but HERES THE PAGE" in the episode description the next time I update, rather than sticking a filler episode in the comic itself to announce the missed update. Some folks have posted bonus stuff that went over well (like if you have concepts or extra world-building info that you can sketch up more quickly than you can create an entire comic page), but as a reader I tend to find that getting notified for Updates That Aren't Actually Comic Updates is frustrating if they happen more than, like, once. And I feel like trying to do extra stuff to "make up for it" can sometimes just put you further behind!
On sites like Tapas and Webtoon, I'm subscribed! I'll be notified when it updates again -- it's not like a comic on a personal website where I'm gonna check the site and be disappointed if there's nothing there and maybe forget to check back; I'm just gonna wait until I get the lil' notification.