I don't know what happened. Maybe it happened after reboot, but I don't know....file proftpd.vhosts.conf is empty !
How could it possibily happen and why? What should I do ?
If you have all shared IP's then an empty proftpd.vhosts.conf file is fine. VirtualHosts are only required if you have an IP address that's owned by a user. It will use the default /etc/proftpd.passwd file otherwise.