A B757 FLT CREW DEVIATED SLIGHTLY FROM ARR COURSE WHEN THEY LOST NAV INFO FROM THEIR FMC DURING REPROGRAMMING.

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JUST AFTER PASSING KESSEL VOR; THE NEW ATIS CAME UP INDICATING THAT RATHER THAN RWY 1L; WHICH WE HAD EXPECTED; THAT WE WOULD NOW GET RWY 19R OR RWY 12. THE CAPT REPROGRAMMED FOR RWY 12. UPON EXECUTION; THE BOX DUMPED OUR ARR. WE HAD BEEN TOLD TO CROSS 5 WEST OF DRUZZ AT 11000 FT AND 250 KTS. I MENTIONED THAT WE HAD LOST OUR ARR. THE CAPT ATTEMPTED TO GO TO RAW DATA WHILE I ATTEMPTED TO REINSERT THE ARR. THE ARR PLATE NO LONGER GIVES A FREQUENCY FOR THE KESSEL VOR! THUS NO RAW DATA COULD BE USED. I HAD NOT YET FINISHED ENTRY FOR THE ARR; BUT REALIZED THAT RWY 12 IS NOT AVAILABLE ON THAT ARR. AT THAT POINT; THE CAPT HAD STARTED TRYING TO GET THE BOX UPDATED WITH OUR CURRENT ARR. WE DECIDED TO ASK FOR DIRECT DRUZZ; WHICH THE CTLR WOULDN'T GIVE US. DURING THIS PROCESS; I ERASED WHAT THE CAPT WAS ENTERING USING THE NUMBER 2 BOX. OOPS. WE MANAGED TO KEEP THE ACFT ON A REASONABLY ACCURATE TRACK USING HDG SELECT UNTIL WE HAD GOTTEN THE FMC RECONFIGURED. THE DSCNT WAS MADE ON PROFILE; BUT THE ENTIRE SITUATION WAS LAUGHABLE. AFTER WE LANDED; WE DISCUSSED WHAT HAD HAPPENED; WHAT WE COULD HAVE DONE BETTER; AND FINALLY JUST LAUGHED AT THE STATE OF CONFUSION THAT THIS FMC ISSUE CAUSED. I DO NOT WANT TO SEE ANYONE ELSE GO THROUGH THIS WHEN DULLES IS IFR AND THE CTLRS ARE NOT PREPARED FOR AN ACFT DEV FROM THE RNAV ARR. I HOPE THAT THIS ISSUE WILL BE ADDRESSED.