My Panasonic 42" plasma, px77u, suddenly turned off and won't turn on. It's 8 years old - worth fixing?

Plasma still has an excellent picture so it depends on whether you want to get a bigger TV anyway and if can do some work yourself. Sounds like the power supply is bad. You may be able to find a replacement board or rebuild service on ebay. A repair tech would probably charge you $200 to do the repair. Your call.