Hi Cheapshoes,
If you can't work out which bit is broken, then simply assume the whole of FMGC1 or FMGC2 is US and apply the appropriate procedure.
I've had the FMS part fail once during flight - but the AP could be engaged in "simple" modes like HDG, VS, OPEN CLIMB / DESC, ALT - all with manual thrust.
It was like a good day out on 707s.