When you reach your storage limit, the extra videos you create will be grayed out and placed under the "All my unavailable content" section in your "My Content" library.
You won't be able to assign these videos to your students unless you increase your storage space or free up some space. As you remove old videos from your account, your unavailable videos will be added to your usable content, from oldest to newest.
You can also check out our article on how to increase your storage space.