It happened recently for some webcomics, they were just stolen and roughly translated in another language iirc. It was a website platform who stole it from the authors.
The platform closed a little bit after, I think? I couldn't get into there so I think it closed.
So, what can you do against it besides report it? Get people to complain with you XD
Seriously, it's really hard to protect your work on the internet, but you should get into your country rights. For example, I released a paper version of my comic and I sent one copy at the Bibliothèque Nationale de France (the biggest library here), and my work is protected by the laws. I can prove that I'm the author by showing the psd files, the dates of the files, etc...
You can also leave a signature at the end of each apges if you want. Some creators do it. You'll always leave a trace on your work, and if people reupload it, there may still be your name on it.
There's also your friends who can warn you about those kind of problems too, or even fans. Or you can check on google image if a page is used somewhere, etc, etc...
There isn't one big solution that can erase every problems for protecting your work, but at least you can try your best to do it!