LG monitor in 'power saving' mode

Hello,

I just bought a 2nd hand PENTIUM 4:2, 512Mb RAM, 150Gb HDD, with an LG 15" SCREEN.

When we switched it on at the buyers house, it worked perfect 100%.
Now, putting it back together and switching it on at home, my LG monitor stays in 'power saving mode'.
HELP!

I have head this cab be due to the power source not being powerful enough- how likely is this to be the problem? It is not the cord, as the screen worked fine at buyers house.

Any help would be greatly appreciated :)