Cash only and bank failures possible?

[homerunhitter]

Hello,
I love the vast knowledge and great wealth of experience you guys have here so I ask this question (because I know you guys will give a honest, straight forward answer) based on what I am reading on the internet, my question is:

Do you think if china, russia, iran, etc decides to have big balls and attack our power grid, will that have an effect on our banking systems and us having access to our money. is this likely? I ask this because lets say our banking power grid goes out nationwide, and we dont have access or the ability to go to an atm and get money out (this could be for days, weeks or even months in a worse case scenario) how do people survive if their bank funds get turned off. or if electronic cash registers lose power. how do you pay for things like groceries? do you recommend maybe going cash only so in case this happens you would have cash on hand? again, how likely of a scenario is this and is cash only the way to go? Will cash always be king?