Ouch... it could be any number of things (bad video card, cpu, motherboard, etc). Do you get any beep codes from the motherboard when you turn it on? Also, does it sound like the system is booting into Windows?
If you get beep codes, come back in here and tell us exactly what they sound like.
You are already doing a good job troubleshooting by checking the monitor on another computer. We know the monitor is good. You might as well check the cable the same way.
Unfortunately you may have and even worse problem than you video card. Can you swap video cards with another computer? If so you can determine if the card is good just like you already did with the monitor.
Do you have a modem and did it just storm? If so you might have got lightning hit and your modem might be dead and it might be taking the rest of the system with it. If you determine your video card is fine you will need to look deeper. If your unplug everything that you don't need (like modems, soundcards, floppys, hard drives etc) and see if you can at least get a bios boot screen to show on the monitor.
Can you swap ram and or CPU's with a friend and see if they are good? Keep in mind you may end up burning up 2 systems so you are taking a chance. I've had pretty good luck with bad parts keeping local and not cascading and burning up other parts but is always a possibility.
Not knowing your system.
Is the 9800pro and AGP or PCI-e?
Is the rivatnt an AGP or pci?
If they are plugged into different slots this will narrow the problem down to the port being bad or the video card being bad.
You should contact the manufacture of the card and try and RMA if bad.
I had the same situation and it turned out that the +5V input had destroyed one of the traces on the board itself. Look very carefully at where the aux. power input plugs into the board.
I was able to fix the issue by soldering a new wire from the input to where it terminates on the board. From what I was reading on the net when it happened it seems to be a common issue especially with ATI made boards.
Retail Built BY ATI 9800 pros have a 3 year warranty. You can check online by serial number to see if it's in warranty. Many other powered by ATI brands only have a 1 year warranty. You'll have to find the website on the box, or just google it.
If your card was made by ATI then you could have a three year waranty on that puppy. Go here to see if they will give you waranty service. The process is very easy. I've done it twice for my ATI AIW 9700 Pro. When you get your RMA number you'll have to mail your card to them at your expense. They'll take about a week to "fix" your card and then they'll overnight a reconditioned card back to you.