By FMC I assume you mean FMS....like loosing your GPS, or GNSXL....
Um....well if your going direct...and now you have to fly by VORs, ADF, DME, ect....not an emergency...your just going to have to start pulling out charts...and using other means of navigation....
Going from SFO to Hawaii, would probably be an issue...
I always bring a little hand held GPS with me....