All the enemy has to do is hit our electrical distribution system.
Without power, no lights, no computers, no air or road traffic control, no water distribution, etc., etc.
Russians use Ukraine as their cyberwarfare testing ground. They have already partially shut down their power grid which included a nuclear power station,
have crashed their flight booking & display systems & shutdown bank's IT systems including ATMs. In the US and many other countries, including the UK, the Russians (and probably the Chinese) have penetrated the power distribution systems & installed malware to take control as & when.
A real game of cat & mouse going on here which if successful will cost lives.
Alternatively, grid shutdown maybe caused during the winter with increasing numbers of intermittent, mass bird slaughtering wind turbines & also the sun not shining due to thick overcast skies means all power grids are now much less stable. Where EU pollution/CO2 rules mean most of the UK coal power stations have been shut, due to this, our winter demand excess capacity is now around 0%. Australia have had two large grid shutdowns like this & restarting a grid is not easy.