There is enough fuel if we are smart about it. Grow trees, process into woodgas, reuse old infrastructure. Add other kinds of biogas into the mix. Use old hydro ponds and rivers too. Geothermal where it's available. Wind generators are trivial to build. Solar thermal energy can give a lot for relatively minimal engineering effort.

And then some time after the apocalypse - nuclear energy. If the knowledge is preserved and security/regulatory standards lowered, the engineering itself is not too complicated. It's likely that some of the old nuclear reactors would survive the apocalypse.

The path from wind powered generators to an Intel 4004 is long, especially without plastics, which we would never develop and for which all alternatives are still, with today's technology, very limited

Realistically we probably wouldn't even need cpus and would have much more important things to work on, good luck smelting iron with biogas...

Nuclear power plants need constant maintenance, France had to shut down a good parts of their reactors during covid because they couldn't work on them due to covid restrictions, and that's very far from an apocalypse, let the rot 100 years and they'll be unusable

Doing what we did without virtually unlimited free energy is like sending a kid to climb the everest, naked, before you taught him how to walk. Petrol is the backbone of everything, medicine, agriculture, metallurgy, &c. We haven't found alternatives for a lot of use cases and that's with out current tech, a reboot would be game over