Sleep mode

Dear all, I'm new here, but would be greatful for any advice anyone can
give.

I'm using an nVidia Ge-force 4 MX 440 with a Philips 170C4 flat panel
monitor. Ever since I bought the monitor, when WinXP's power settings
powered the monitor down, it went into sleep mode. Since the beginning of
the week, I've noticed that instead of the monitor going into sleep mode, it
now states it is, but then throws up a "no signal" message. I've not
changed any settings or anything and this has just happened all of a sudden.
Can anyone advise as to whether it's the screen or is there anything I can
play about with with the video card? All leads, etc seem to be fine, so
it's not that obvious! I've followed all troubleshooting stuff available
from Philips for the screen, but no joy.

I appreciate you might need more detail than this, but thanks in advance.

Spec is:

Shuttle AV40 MoBo
768 MB DDR RAM
60GB Nikimi HDD
Liteon CDRW
Atapi DVDROM
Floppy, etc, etc.