A supernova in the Alpha Centauri system has sent a burst of radiation that has disabled most electronic equipment. The modern world is not functioning very well. No phones, no radio, no computers, no transportation that depends on electronics...
In particular, we learn about three people who have been treated for cancer with experimental therapy. The medical office where they were treated has been looted and the doctors are dead. The treatment was new and not widely available. And there is no electronic communications to try to locate any. They can only think of one way they might get further treatment. They know of a brilliant doctor who played a role in developing the technique. He's serving a very long sentence in suspended animation for being a serial killer of girls.
They manage to get Dr. Guest out of the "jail" and make him work at a laboratory in the doctor's old house, putting together a comparable treatment. Needless to say, Dr. Guest does not want to remain their prisoner forever, or for them to get rid of him. He would rather return to the activities that ended up getting him put away. As we progress through the book we get excerpts from Dr. Guest's diaries, which suggest he is still around.
We also follow the crew of an expedition to Mars returning to Earth. Their spaceship was not designed for re-entry and landing. They were supposed to transfer to a space shuttle in orbit, but there are no working shuttles. They don't have the resources to stay in space indefinitely. They must use their knowledge to find a way to get down alive. When they finally do get to the ground, they find themselves captured by an armed end-of-the-world cult with plans to seize power.
Meanwhile, the bright supernova is disrupting Earth's climate (especially the southern hemisphere, which faces Alpha Centauri). The US president is trying to hold the country and international community together as best he can, while fending off political opponents. Not to mention the armed cult.
...and it turns out the supernova is sending a barrage of slower-moving particles towards the Earth which will cause a worse crisis in 50 years.
Half of the book is more "disaster fiction", but there are also good science elements.