Sorry, I was really worried and I forgot to give more necessary informations.
In the mentime, I partially solved the issue, because I understood we have an access in "Serial Console" mode, and in this way I've able to reach my virtual server, and reactivate the ssh function.
Issues are begun surely when I tried to changes some permission with chown and chmod on many folder, in order to try to activate a new service.
I've been able to reactivate even the ftp connection, but now still the Plesk error, and I'm not able to find which permission I need to restore for this service.
Any idea? We are speaking about a Centos 6.4 server and this is the ls -alt situation from the root:
drwxrwxrwt 3 root root 80 Mar 6 10:23 tmp
drwxrwxrwx 8 root root 4096 Mar 6 10:04 home
drwxr-xr-x 82 root root 4096 Mar 6 07:07 etc
dr-xr-x--- 5 root root 4096 Mar 5 13:45 root
drwxr-xr-x 21 root root 3760 Mar 4 22:36 dev
drwxr-xr-x 27 root root 4096 Mar 4 22:36 .
drwxr-xr-x 27 root root 4096 Mar 4 22:36 ..
-rw-r--r-- 1 root root 0 Mar 4 22:36 .autofsck
dr-xr-xr-x 13 root root 0 Mar 4 22:36 sys
dr-xr-xr-x 163 root root 0 Mar 4 22:36 proc
drwxrwxrwx 2 777 ftp_matica 4096 Mar 4 14:49 ftp_folder
-rwx------ 1 root root 4257699 Mar 3 11:21 academy.sql
drwxrwxrwx 24 root root 4096 Feb 24 12:09 var
drwx------ 2 root root 4096 Feb 24 10:19 .spamassassin
drwx------ 3 root root 4096 Feb 24 10:19 .pki
-rwx------ 1 root root 0 Feb 24 09:56 .autorelabel
dr-xr-xr-x 5 root root 4096 Feb 24 09:48 boot
drwx------ 2 root root 49152 Feb 24 09:48 lost+found
drwx------ 2 root psaadm 4096 Feb 24 09:48 archives
-rw-r--r-- 1 root root 160 Feb 24 09:48 migration.log
drwx------ 2 root root 4096 Feb 24 09:45 bin
dr-xr-xr-x 2 root root 12288 Feb 10 09:12 sbin
dr-xr-xr-x 9 root root 12288 Feb 5 08:50 lib64
drwxrwxrwx 9 root root 4096 Nov 24 11:05 lib
drwx------ 2 root root 4096 Oct 18 01:12 cgroup
-rw-r--r-- 1 root root 970 Jun 8 2012 monit.service
-rw-r--r-- 1 root root 1125 Jun 8 2012 wdcollect.service
drwxrwxrwx 14 root root 4096 Dec 19 2011 usr
drwx------ 2 root root 4096 Sep 23 2011 media
drwxr-xr-x 2 root root 4096 Sep 23 2011 mnt
drwxr-xr-x 2 root root 4096 Sep 23 2011 opt
drwx------ 2 root root 4096 Sep 23 2011 selinux
drwx------ 2 root root 4096 Sep 23 2011 srv
Thanks a lot for your help!