To add to the previous posts on cyber attacks (helping protect critical UK systems from this is part of my day job)...
999 systems could be hit. Meaning calls to the emergency services go unanswered. Limited police response because it all falls back to someone, somewhere writing down each call and trying to find who is free to respond (if the phone lines themselves are not hit - which they may be).
If they hit those, they can and probably will hit crime recording and evidence databases, meaning criminals cannot be found, arrested, charged or convicted.
NHS systems: when you hit those, wards need to close. This means people with life threatening medical events/illnesses have no where to go. Or, if the wards are allowed to remain open, limited care given because machines don't work. Surgery goes back to being analogue which reduces the types of surgery you can do and the people you can save - with younger surgeons having to learn whole new ways to operate because they have always had cimputers to help and guide them in complex cases.
Banking, communication, councils, the military, the government, police, fire response, medical response, water and energy supplies, food supplies, all vulnerable.
In short, everything requiring technology can be impacted and - as others have said - it is true that:
a) this is easier to pull of than the public imagine (a healthy dose of luck and relative geopolitical stability is what has previously provided defence)
b) it will literally take weeks, months, even years to bring some of these systems back. Both hardware and people resources will be in very short supply and there will be long queues for both.
The cyber question, is a big deal here. We have succesfully fended off those attacks but, genuinely, our defences are not that good and luck has played a large role here. That and the fact that the attacks either come from non-government sources are need to appear as if they have. So they are smaller scale, mostly. They are just out sniffing for a weakness but otherwise often not especially malevolent.
Open economic warfare with government-sponsored Russia cyber organisations prepared to come out of the shadows and target all Western countries? Different ballgame. It wouldn't be a walkover but it also is a real risk.