X200 Sleep problem

[duncanmcload]

I bought and X200 with Vista 64bit. I have a question about power settings. After an idle period first it starts showing screensavers. I can resume from this state by pressing a button, so it is OK. However, after a very long idle period after showing screensaver, it shuts the screen. I can no longer resume from this state. I tried pressing keys like power button, Function, Function+F4 but there was no response. I can feel the HDD or fan is moving. Pressing power button for a long time restarted the computer. How can I resume from this state?