Error -3278

[s.tomanek at gte.net]

My iMac Airport Indicator (the five green dots on the control strip)
suddenly turned grey and I am no longer connected to our home DSL local area
network. The error message says "An error (-3278) occurred while
communicating with the AirPort driver."
Does anyone know how to fix this?
I've opened the Mac and checked the Airport card isn't loose. I tried to
reinstall the AirPort software but get the same error message when I hit the
Start button.
My PC works fine.