Do to the way the Gutenberg Block Editor manages blocks there are times when a block may appear broken. This can happen for various reasons and if it’s repeatedly happening to you then you should get in contact with the block creator to determine a long term solution.
What appears is a warning message “This block contains unexpected or invalid content”. And your block will look like the image below in your admin:
It looks like you only have two options and both usually lead you either no content or an html output of code which can’t be edited. But with the latest versions of Gutenberg, there is a very handy hidden tool in Gutenberg that allows you to attempt a block recovery.
FIRST: go and install the Gutenberg plugin, this will update what’s being used from core WordPress and allow you to access the latest features. You can go to Plugins > Add New and then search for Gutenberg and install it.
Once installed go back to your page and click on the more options icon button and choose “Attempt Block Recovery”.
We’ve found this to work almost every time.
I hope that helps!