In his SF novel "Dies the Fire", S. M. Stirling posits some event that stops all electronic and most chemical technology from working. The 21st century world is quickly replaced by a medieval one.
Yet Stirling is a pretty bright guy, and he has some bright characters, so a few of them wonder at the subtlety and precision of the event, such that it stops gasoline from burning quickly, and gunpowder from working, and yet allows human brain cells to keep processing information at exactly the same rate.
As you might guess, Stirling never reveals exactly what fundamental constant became a variable, nor how much it changed, but that's to be expected.
It is fiction, after all.