Card Authors "close" their Cards when they need to take them "offline" for maintenance. While closed, a Card's badge will display a small black wrench icon, like the following Employee Onboarding Card:

While closed, existing enrollments can continue but no new enrollments can begin.

If you're a Learner who wants to enroll in a closed Card, you'll have to wait until the Author re-opens the Card. When that happens, the black wrench disappears and looks like the following:

You can also set up a notification in your Profile to alert you any time a Card opens.