If you reach the traffic quota for your hosting account, the website will display a message that your account is over the quota. When your account is over the quota FTP access will be restricted (The File Manager section will also be disabled).
Note: The traffic counter is reset at the beginning of each month, so on the first or second day of next month, your website and files will become available again.
If you would like to remove the monthly traffic restriction please consider upgrading to a paid hosting plan.