First off, don't worry! Your account content is still safe!
This often happens when a user is trying to open Planboard or other Chalk applications on an outdated browser. Our Product team always works to deliver a rich user experience when it comes to updates and features. In order to do this we often require the most recent Chrome and Safari updates, as this allows us to meet all of our user requests and needs to make your job easier!
In order to fix this, all you need to do is update your Chrome browser. Here is an article by Google on how to do so!
If you're using Safari, here is Apple Support's how-to support article.
Once you are all up to date, you can restart your browser and everything should be good to go.
Happy Planning! :)