Did your cards on your site do something like this after last Tuesday's release?
Up until about a month ago new sites had a block of HTML examples that contained a "heading row" HTML element <hr> that was not being honored by the site layout engine. If you made a site recently you won't see this issue because we updated the site templates to use the better calcite bootstrap card examples which don't have this HTML element.
As of last Tuesday we made <hr> tags function in Sites so you have additional layout options.
If you have used these old card examples on your site then you need to make an update to fix the layout.
To fix this issue remove the <hr> tags from the offending cards
...once you do all will be well:
We apologize for the inconvenience!