Double FMGC failure is peculiar I wish airbus had done a better job, in that ECAM doesn't directly tell you what has happened but makes you guess it by the symptoms. ECAM shows CABIN PRESS LDG ELEV FAULT which is understandable but doesn't tell you the cause and NDs show map not available. MCDUs give you a option of BACK UP NAV if installed otherwise I think go blank and off course FMA disappears and you loose all automation AP, FDs, ATHR.