You say you want to go with something accidental, but you can take it even further. What if some people do something they think is helping and it causes catastrophic failure.
A private software firm develops a new program for power grid management. You may or may not be aware of this, but most power grids need constant maintenance to balance supply and demand of power. Too much provided, and the grid shorts. Too little supplied, and systems shut down. As a result, the power plants for each regions grid are constantly changing output. It is inefficient and relies nearly exclusively on human influence. Water flow through dams, burn rate for coal plants, and number of cells active in nuclear plants are all dependent on people, with very little computer influence. What if that changed? What if our software company found a way for a computer to observe supply and demand for power and learn when and why to adjust output, adapting to all forms of generation. This company would instantly become rich, as energy firms seek to use their hopefully patented tech. It starts small, taking root in cities first and then is applied to nationwide grids. It is amazingly effective, causing the energy industry to reduce failures and increase reliability.
It was a simple mistake. The software firm is working on an update to their system, one that energy firms are deadly waiting for. When it comes out, all of them want it, and they allow some of their newly computerized systems to accept the update and all goes smoothly. They wait a week, a month maybe. All seems well. So, they push the update to more systems it until all of their grids use it. After all, if it didn't fail within a month, it must be safe. And it was, for a time. But unknown to all, there is a glitch in the update, a miswritten definition in one of the program's library files. This particular file isn't called on by the main function very often, so the glitch remained unnoticed. Then, one day, catastrophic power failures everywhere. Coal plants stop working. Dams are frozen at their current flow rate. Nuclear plants completely shut down, their cells grow cold. The lights go out. The world is dark.
Basically this idea depends on a glitch in computerized grid maintenance causing a global shutdown. This idea, scarily enough, is plausible even today. I'm sure the energy industry would love a way to manage the grid without constant human attention, and a bad update is something everyone deals with eventually, from iPhones whose hardware can no longer handle the new iOS to government websites with broken links.
Hope this helps.