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!