Acer monitor "Cable Not Connected" message

While over my daughter's house and in using her computer, turned it on and while warming up, the Acer 243W monitor screen gave a, moving over the screen in a gray box, "CABLE NOT CONNECTED" message. The computer and monitor are working OK, surfing, tasks, video games. Regular cable tv reception not that good. She said it's been that way since her newbie husband put it together. Can't ask him, he's vacationing in Afghanistan for the next 11 months.

She has a Radeon 4870 VGA , Vista 64 ultimate, and a TV card. The monitor has 4 inputs on its bottom. The power cord plug, HDMI in plug, DVI-D in plug and a VGA in plug. Only two are being used: the power plug and the HDMI in plug, with the HDMI connected to one of the 4870's VGA connector ports, the one on the left side when facing the back panel.

Got Acer's site and their manual for this 243W installation show directions that state 1-1) "Connect the VGA cable to the computer"; 1-2) "Digital Cable (Only Dual-Input Model) - states"connect one end of the 24-pin DVI cable to the back of the monitor and the other end to the computer's port; 1-3) HDMI Cable (Only HDMI-Input Model) - Connect the HDMI cable to the computer. Per their site this 243W is a HDMI monitor.

Is this monitor connected correctly? If it is any ideas on why the message, "Cable not connected". I'm truly unknowledgeable in this area.

Thanks for any help.