You should have the evacuation information in your H&S system. Much of it will be specified by the local authorities under their H&S legislation. You will have emergency lighting so your workers aren't in complete darkness won't you?
It's the recovery that will be a challenge. You'll need to complete all your start up procedures before re-starting, including calibrations and labels. Check all temperatures as your chillers/aircon etc will have been out of action. Clear the lines of product as you cannot guarantee it. Ensure that the correct recipes are loaded into the machines, that your date coders have restarted correctly and have been reset. There's probably more that others can add for you.
I've experienced a few of these and they're no fun. You know you've lost production and time, but you should never hurry the restart even though the production dept would like to.