Laptop won't wake up from sleep mode

I have a Dell Inspiron E1505 running Windows Vista. I am using it as a desktop replacement with an external keyboard, mouse, and monitor. I have the monitors set up so that the display goes to both the external monitor and the built-in monitor and the external monitor is set up as monitor 1.  Whenever the system overnight (or for long enough to go into sleep mode) the monitor turns off but neither the internal monitor nor the external monitor will turn back on when I come back and try to get the system working again. I end up having to hold the power button down long enough to turn the system off and let it reboot.

How should I have this set up so that it will work properly? I want to be able to let the machine turn the monitor off and go into sleep mode -- and then have it wake up when I come back without having to reboot it.

Thanks,

Johnny