Error MySQL Conection

Hello friends!

I lost the database connection and I no longer know what to do to solve it.

No ee-admin it pops up an error.

phpMyAdmin - Error

Error during session start; please check your PHP and/or webserver log file and configure your PHP installation properly. Also ensure that cookies are enabled in your browser.

session_write_close(): write failed: No space left on device (28)

session_write_close(): Failed to write session data (files). Please verify that the current setting of session.save_path is correct ()

I don’t know how to back up files.

I really need your help!