How Do I Get My Computer To View On My Tv Screen?

I went to radio shack and they told me to hook a vga cable from the back of my tower to the back of my flat screen TV. I did this and when I turned on the computer, it started to show up on the TV in the living room. all seemed to be working well, then, just at the point where I would type in my password on the keyboard, the screen on the TV went blue and in the top right hand corner it said "not support" . what happened? can I bypass it? Help please. 10 points to first person to solve my issue. thanks