As Covid-19 reaches pandemic status and hysteria is in full swing I just reminded myself I wanted to write a quick guide how to survive a month in your flat in a city like Vienna if you can't buy anything. As a healty, slightly overweight couple.
1. You need water.
Most likely the authorities will try everything to keep water on, because if the water is off that would be catastrophic pretty quickly. But for the same reason it is also the most important to secure.
You don't want to shower. You don't want to do heavy excercise. You just want to stay in your flat and not go out for a month. So you need maybe 3L per day per person + 2L for cooking.
So for two people you need 240L. That's 0,24m³ and quite heavy: 240kg approximately.
But you get 3x30L plastic containers for 34 Euro on Amazon: https://www.amazon.de/Kanister-Deckel-Zapfhahn-Ausgie%C3%9Fer-Trinkwasser/dp/B01786V4AS/ref=cts_sp_2_vtp
So that's like 100 Euro for the water storage. Test the containers before usage and make sure they are not leaky.
Fill them up with Austrian safe tap water and add some conservation additive. https://www.amazon.de/Katadyn-Micropur-Classic-1000F-100ml/dp/B000S0I9AU Don't put the 240kg all in the same place. Spread the weight of the containers out over the flat.
2. You need food.
You are in a city and if there is no supply in the super market you quickly run out of options. You need about 2000kcal per day per person. That's 60.000kcal in a month per person.
Cheapest is to buy spaghetti. Spaghetti has 3540kcal per kg and costs 0.8 Euro per kilo https://www.cleverleben.at/produkte/1961/spaghetti
So you need 17kg of spaghetti to survive a month for 13.30 Euro. In the likely event that you don't need it then just throw it away after 3 years or eat the spaghetti before it expires.
That is all mostly cabohydrates and no protein or fat. So ideally you would also have a bit of protein and vitamins. But you will survive.
If you are slightly overweight/obese (read "fat") but otherwise healthy you can easily survive on 1000kcal per day for a month. So you can both share the 17kg of spaghetti with your significant other. In any case food is probably the cheapest of everything on this list.
If you eat Soylent/Mana/Jake/... ( https://drink-mana.com/ ) that is much more expensive, but you have it easiest because you probably already have a month's supply of instant-food with all you need at home. And you don't need electrical power or a gas cooker ( https://www.amazon.de/Campingkocher-Gaskocher-Koffer-inclusive-Gaskartuschen/dp/B00MR34RCA/ ) to cook the spaghetti al dente.
3. You need communication.
You likely already have a mobile phone. And an FM broadcast radio receiver.
If you are paranoid also get a small ham radio https://www.amazon.de/BAOFENG-GT-3TP-Dualband-Handfunkger%C3%A4t-Headset/dp/B00R7OB7T0. 41 Euro. Strictly speaking you also need a license to use it ( https://www.oevsv.at/amateurfunk/wie-werde-ich-funkamateur/ ) and you need to know how to use it and to programm the local repeater. So that's not exactly trivial.
And you maybe want to buy a small solar cell.
https://www.aliexpress.com/item/33002940234.html (65 Euro, need to sign up on Aliexpress to view item)
Test that it can charge your mobile phone and your ham radio and that you have the proper adapers and cables.
4. You need a gun.
Get a gun license for self defense. That costs approximately 450 Euro.
Plus followup costs for renewal every ~ five years.
Affordable pistol and amunition probably 1000 Euro.
And basic gun storage 150 Euro.
5. You need a resilient medium of exchange. (optional)
Consider a gold bar. There are affordable ones where you can break out 1/10 ounce pieces to swap with your neighbors https://philoro.at/shop/goldbarren/gold-combibar-1-oz-philoro
And consider buying some fraction of a Bitcoin to still be able to send and receive "money" from around the world, even if banks are closed, as long as you have some internet access. E.g. buy with https://www.bitpanda.com/ and store on https://shop.ledger.com/products/ledger-nano-x (120EUR) wich connects to your mobile phone if needed.
I wrote this whole guide in an hour with very little research and as always I give no warranties whatsoever. Be reasonable. Don't do stupid things. If you are old, have kids or have medical problems you need to prep differently.