I just woke up.
I think I have given a bad example.
Maybe I should asked "if the staff on sick leave, is it my problem?"
I haven't heard of power plant down due to windows BSOD. At least not in my country. Where do you live?
In case there is a power disruption, we have power backup. Otherwise, I provide a mobile or tablet version. The support is based on SLA and whether customer has subscribed to technical support plan. If still the issue is not solved, customer can raise a ticket. Lastly, store can temporary use manual way. There will be minimum impact. It is better than cloud based software.