Usually when I leave my desktop for a while the screen goes dormant but comes back on when I move the mouse. In fact sometimes it comes to life when I simply enter the room (wierd). However sometimes it doesn´t and nothing I do will bring it back short of shutting the computer down with the on/off button then turning it on again. What´s going on and is there something I can do to bring it back to life with out the drastic measure shutting the computer down with the on/off button?

There are two states that are involved here. Monitor sleep and hibernation. The power saving settings allow you to power down your monitor after a time of inactivity. The monitor can be wakened by touching the keyboard or moving the mouse (its possible entering the room creates a vibration that the mouse picks up). Hibernation is a deeper "sleep". Have you tried pressing Ctrl+Alt+Del when just a mouse movement won't waken it?

No I haven´t but I will try that next time. Thank you.