Let's say you've gone back in time...
No tools, no technology, no resources, no nothing. Although there are many instant ideas you can use (basic physics, survival skills, etc.), one would definitely not begin with creating a personal computer (or a Mac, if you're a fan), but by breaking it down into smaller elements and ideas, you'd have to come to the conclusion that a complex device like that would be virtually impossible to rebuild in the conditions of zero technology around.
The question remains: where exactly do you start from?
I believe that no matter how bad it gets, it is crucial to give yourself a project. The same idea that motivated me to build a sauna on a summer camp, made me realise how important it is to look ahead and work towards a goal.
Thomas Thwaites decided to start with a toaster: