Have you tried resetting the Video Output?
If the PS3 LED stays green and nothing else happens.
Press and hold the power button till the LED goes RED [Off]
Then press and hold power button until you hear a beep then release.
Video will attempt to reset to lower settings.
If you are using HDMI and this still fails i would suggest trying a different HDMI lead or the Composite AV Leads [RED/WHITE/YELLOW] before attempting a major fix like described above.
If you change the leads dont forget to reset the video on successful change