AN SMT PLT MOMENTARILY LOST CTL OF HIS ACFT AFTER INADVERTENTLY TURNING OFF THE AVIONICS MASTER SWITCH AT NIGHT IN THE WX.

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ON DSCNT FROM 9000 FT TO 3000 FT WITH THE AUTOPLT ENGAGED; I ACCIDENTLY HIT THE AVIONICS MASTER AND TURNED IT OFF. IN THIS PLANE; WHEN THE AVIONICS ARE TURNED OFF; THE TRANSPONDER REVERTS TO A CERTAIN DEFAULT CORE (IT DOESN'T HOLD THE PROPER CODE IN MEMORY). I IMMEDIATELY TURNED THE AVIONICS ON AND STARTED TO REENTER THE TRANSPONDER CODE; FORGETTING THAT THE AUTOPLT HAD ALSO BEEN TURNED OFF. WHEN MY ATTN RETURNED TO THE FLT INSTS I REALIZED I WAS IN A SPIRAL DIVE (60 TO 90 DEG BANK AND 4000 FPM DSCNT). I IMMEDIATELY INITIATED A RECOVERY BUT DSNDED AS LOW AS 2500 FT AND ON THE RECOVERY CLBED BACK TO 3400 FT BEFORE REGAINING CTL OF THE ALT. I THEN RETURNED TO MY ORIGINAL HDG AND ALT. I INFORMED NORFOLK APCH THAT I HAD A PROBLEM BUT DID NOT SPECIFY WHAT THE PROBLEM WAS. OBVIOUSLY THE PROBLEM WAS CAUSED BY A LACK OF ATTN ON MY PART. I SHOULD HAVE REALIZED THAT THE AUTOPLT HAD KICKED OFF WITH THE AVIONICS MASTER. I DON'T KNOW WHY I GOT SO CONCERNED OVER THE TRANSPONDER CODE. AT THIS TIME I HAD BEEN UP FOR 24 HR SO I WAS TIRED WHICH LED TO BAD JUDGEMENT ON MY PART. I DON'T KNOW IF THE TRANSPONDER DEFAULTING TO ANOTHER CODE IS NORMAL; BUT IF IT HADN'T CHANGED; I PROBABLY WOULDN'T HAVE HAD THIS PROBLEM.