Disk quota exceeded

When you log int your server you get some error like this:
/tmp/Rs6V6BOv: Disk quota exceeded

Probably cpanel is not running and users cant send files using FTP.

Probabaly you ran out of inodes.

Run this command:
# df -i

And check if any partition is 100%.

Inodes are related to number of files you have under a directory and not to the disk space used, you should be having a large number of tmp files or a huge mail queue.

You can check the tmp files on this file: /var/tmp and mail queue on this /var/spool/exim/input/