My PS3 HDMI isn't working on my TV?

So I have this new Panasonic TV. I hook up my AV cable to my PS3 and then I hook up my HDMI cable, it tells me it detects and HDMI cable and would like to know if I want to switch. I say Yes, screen goes black. I switch to HDMI and nada. I don't know how but I got it to work once about 2 weeks ago but I can't remember what I did, and why it happened again. And yes I've tried resetting the video settings by holding the power button in, that's the only way it lets me view it in AV again.