Self test, check your PC and signal cable, monitor is workin

Hi all, I bought a 2nd-hand Windows XP eMachine PC a week ago. It worked OK for a few days then when I went to turn it on yesterday morning I just got a message on the monitor saying, "Self test, check your PC and signal cable, monitor is working".

The PC seems to be on but all that shows on the monitor is the above message.

I tried unplugging everything and plugging it back in to no avail. Thats the extent of my skills level. (If you can access my previous thread, you will see that thinking of turning off and on again is actually progress for me.)

Anyone any (polite) suggestions?

thanks