Hdmi handshake problems unfortunately. My PS3 has been very problematic in this regard whenever I take it out of my system. You can try power-cycling your equipment or differerent hdmi cables, but that has never worked for me. What has worked is swapping out my PS3 for my 360 (seems to re-establish the connection) and back again. If you don't have a 360, I imagine any hdmi source component might do the trick. If that doesn't work, you might have to use component cables (the first time it happened to me, I dug out my old PS2 component cables which are compatible with the PS3), but I know this is a less than ideal solution.
You can also try getting into the PS3 Recovery Menu ( hold power button until it beeps twice) and restarting the system.