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.

thanks