Desktop won't lock

[jeenuv]

This is a behavior that I've been noticing recently on Xubuntu 12.04 on my Asus X55C: normally, when I leave my laptop unattended, the display switches off according to the power manager settings. But , after some time, the display just wakes itself up and won't turn off again; it just goes blank. Also I would no more be able to lock the desktop with Ctrl+Alt+L key.

Any clues as to what's going wrong here?