If google crawls my site and its down what will happan, will it come back, if im unlucky will it mean I lose my rankings, say my sites down for a few hours what happans, and say a few days what happans.

Is it pot luck in that you hope google doesnt visit when your sites down. Im doing some work on a large site that will require it to be down perhaps, we are worried what effect this will have on our rankings, is there anything we can do to help it.