Google it, there's a ton of other forum posts out there. Something about either the output IC or the fans.
I bought a used 5.1 Panasonic SA-HT540 home theater today and the first thing I did was to plug in the subwoofer and the center speaker just to make sure it could actually play some tunes.
When I connected the rest of the speakers I heard a noise coming from one of the surround speakers so I turned off the DVD to check all the cables. When I turned the DVD back on I was greeted with "F61" showing on the screen followed by the fans and the unit turning off.
I unplugged all the speakers and even cleaned out the two fans but it keeps giving me the error code and won't start.