|No you only need one cable. You might check the monitor manual to see if there's anything special you need to do when using either connection but you probably just need to make the connection and the monitor will auto detect which one is being used.|
Or just temporarily connect it to the computer you're using now.
After you've verified the problem is not with the cable take a look at my # 2 post above.
I assume you're not getting any posting beeps, right?