Quite an issue many PlayStation 3 users are experiencing after switching cables or TVs. I've experienced the same problem after buying my PlayStation 3 off someones hands.
If you recently switched from HDMI cable to A/V output (for older televisions) you'll notice that your PlayStation 3 is working fine but there is no display on the screen, the reason being is that you haven't changed your video settings to lower or higher resolution depending what you're switching from. But the problem you're experiencing is that you can't switch the video settings on your PlayStation 3 console because there is no display.
To solve this, all you have to do is plug your console in and make sure the red light (stand by) is on. At this point push and hold the reset/power button until you hear two (2) clicks. Doing this procedure once may not be enough, so do it several times but give a few seconds in between for the display to pop-up. First time may not work as your console will just start up, but turn it off and try again. This solved my problem and will yours as well. You either have to change the settings on your console before you switch TVs or perform this procedure after you switched TVs or even cables at times. Enjoy and happy gaming!