Cannot get signal to monitor - No DVI Signal

Mac Pros are not PowerMacs. And PowerMacs are not from 2009 and PowerMacs don't run Snow Leopard (Mac OS X 10.6). So let's get our facts straight so the advice can be concise. Please use this article to verify what you have:

Do I have a Power Mac?

Not that it really matters which machine you have, both PowerMac and Mac Pro could be connected to any digital display with DVI or ADC with the right adapter. The two most common graphics display connectors on these Macs were ADC & DVI:

The Asus 248-p has a DVI connector. Now mind you, if you look at this image:

You'll notice that DVI-D is different DVI-I. Apple uses DVI-I. You need to make sure the cable in question converts the right DVIs to each other. So look carefully at the connector on the display. The DVI-D is on top of the image, where DVI-I is on bottom of the image of the graphics card.

Notice how ADC has a rounded exterior connector all the way around, where DVI is more flat on one part of the width than the other.

If you do have the correct cable, it is also possible the display card is loose, or that the PRAM battery needs replacing.

a brody pointed out those differences because your post found its way to the PowerMac Forums, not the Mac Pro forums. Your equipment profile also says PowerMac. Mac Pros and PowerMac G5 are alike onbly on cosmetics; they are very different when it comes to troubleshooting.

so this worked before?

The message "No DVI Signal" produced by some displays should be read as "No useable signal, within the parameters of this display."

Sometimes the problem is caused by a display set to a wonky resolution, and there is an approved method to get it back.

Boot to Safe Mode. Hold down Shift at Startup, using a wired keyboard connected directly to a port on the Mac Pro chassis, not on a display or other USB Hub. Your Mac will take five minutes or so to do a Disk Utility (Repair Disk) and show you a progress bar. Then it will ask for your Username and password, even if you normally auto-login. Only a minimal set of Drivers are loaded, and the display (sans Driver) is set to a One-size fits all resolution and will be slow. Set the resolution again and save changes.

Then when you re-boot normally, it will take slightly longer as it rebuilds the kext cache. But you may be back to normal.

Could be a bad graphics card. Do you know what card is installed? A GT 120 was a common one installed.

Did that card work before in this Mac? I ask since this previous discussion says that your car (which was a standard in the 2006 Mac Pro) will not work in a 2009 Mac Pro

Is the NVIDIA GeForce 7300 GT video card compatible with 2009 Mac Pro?

Probably a bad graphics card. Those cards fail as they get old. Have you tried the other DVI port?