Ubuntu 12.04 monitor goes to sleep and won't wake up

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http://askubuntu.com – We have about 80 systems running ubuntu 12.04 64bit desktop edition. We are facing a strange problem. Everything works fine except that after the monitor goes to sleep, if we don't wake it for long time then it stucks and won't wake up even if we keep pressing any key. But all tty consoles are working. Also i checked the syslog and it was showing the following warning message: lightdm: pam_ldap: could not open secret file /etc/ldap.secret (No such file or directory) Also if i pull out the power cable of monitor and insert it back (once the monitor LED goes from orange to black), the screen c (HowTos)