there are some programs that can actually block suspend modes for computers.
I have written software to detect and fix this problem actually.
Windows boxes in power management have a suspend time out. you can set the action for what happens when this "idle" period happens with windows powermanagement. so it will force the machine to sleep , power off, or do nothing. monitors can only "sleep".
some programs cause the sleep message to be blocked. so then the action never occurs. i know uTorrent is one of them, and so is the little tray thing for some dvd player apps (it hink win dvd for one) . so maybe sure you havent installed some of those piece sof software lately. utorrent i know you can shutoff that feature (since they dont seem to have coded it quite right).