Make sure the cable is connected all the way.
Also, boot the computer up AFTER you plug it up to the TV.
If it's still not right, try going into your display settings and see if there is a TV output option. It's been a while since I have set this up, but when you get it working , it's great. I have a networked computer connected to my TV and stereo receiver. The computer has a DVD-ROM drive, and I ended up getting rid of my VCR, my stand-alone DVD player and CD player, all of my DVD and VHS movies. My entertainment center is so clean now, a lot less plugs and wires. This is really cool if you have a large mpeg-4 collection on hard drives. LOL, my file server has over 700GB's of space, and my three systems combine at over 1.2TB's.