The big stores have a plan in place. I worked at a store through college and we had a couple power failures and a system failure.
The cashiers instruction is to keep the line moving and customers happy. So after a few minutes of waiting for the system to come back up a manager gives the ok to go into manual mode. The store switches to backup generator which can run for a few hours and the registers act as basic adding machines. The managers from various departments are called up front to help with any price disputes but unless its a large ticket item you make the prices up as you go. We jokingly said that those were the biggest sales they have since your taking customers words on prices but they want the customer back another day so they are willing to take a short term loss.