Well I've had this problem when I took my PS3 over to my grandma's house and played it on the HDTV there, when I got home it didn't work on my SDTV in my room with normal cables.
So I bought my PS3 downstairs to my HDTV and I bought my SDTV downstairs, I plugged the HDMI and the SDTV cables into the PS3, firstly using the HDMI output, I went to the display settings menu and changed the output to the SDTV, so both TV's were running, both cables were in and then the display changed to the SDTV and it worked again.