808Mom

I got a new desktop computer for Christmas. I disconnected my old one and set it aside for a few days until I could buy a new monitor so my children could use the old one. I bought the new monitor and hooked up the old computer in the kids' room. The computer starts making noises and the lights for the floppy drive, DVD and CD rom drive turn on, but the screen only says "no sync". Thinking my monitor was bad, I hooked up the new monitor to the old computer and got the same thing. I opened the case to look inside but everything seemed to be intact. I found a site which took me step by step through things which might have gone wrong, and I tried re-seating the memory and changing the CMOS battery, but it still does not work. This computer is about 8 years old (Compaq Presario 5423). Help!