There are many chances that you may have left your PC on at one time or the other, without any activities being performed on it, usually in such situations the PC may go into automatic standby, if you have set it up in the Control panel power options.
However if you want to completely turn of your computer and save some energy, a handy freeware called All Off can be pretty useful.
All Off allows users to setup a watch to monitor your system and turn if off if there is no mouse, keyboard or CPU activity for a certain period of time.
You can setup the time value for checking idle time from between 5 and 1800 seconds (30 minutes). The shut down type can be configured to either Power Off completely, Hibernate, close the monitor, reboot, shutdown or standby.
All Off is free for personal use, however you will have to register on the developers website before you can use it.