Electricity.
If you lose electricity most people lose access to:
Hot water
Running water (if you have a well)
Air conditioning
Indoor heat
Television
Internet
Indoor lighting
And hot meals if you don't have gas.
Losing electricity would cut you off from almost all of your luxuries as we've become completely dependent on it over the last century or so.
Yeah in the modern age internet access should be considered a necessity. There are a lot of things you can't do without the internet (like get a job or pay bills).
It does take a bit of preparation for the lifestyle that we are not ready for. Ways to store and prepare food, maintain temperatures, get information, illuminate spaces.