I Am Running Out Of Storage. What Should I Do?
The storage indicator that you can see in the top bar (just below your user name), and on the settings page, warns you before reaching the storage limit. Once you go above that limit you won't be able to send more files.
Note that in this case other people can still use your "Receive files" page, but you won't be able to download received transfers. To get back on track you need to reduce the amount of stored data by removing some of your transfers: Read how to clean up your data.
Alternative solutions to this include:
upgrading your account (if you are on a Basic or Personal plans);
extending your storage (if you are on a Pro or Business plans);
getting the quote for a custom Enterprise plan.