If you check the Statistics for your account in the client area, or the sidebar in the control panel, you will see that your account currently exceeds the inode limit for your account. Inodes are the total number of files and directories of your account. If you reach this limit, you will not be able to create or upload any new files to your account.
To fix this limit, you will need to reduce the number of files and directories on your account. Alternatively, you could consider to upgrade to premium hosting, where no hard inode limits exist.