As half of the the nuclear power plants are down, they have to rely on gas. And gas has become difficult to import.
France feels both the liberalisation of the electricity production : Private operators promised to produce cheaper and cleaner power back in the 2000s. But they didn't. Virtually all of them used a scheme (the Arenh system) to get public electricity in bulk, sold to them at a loss and at a fixed price, so that they could compete with the public company on retail prices. Then the government was still in their privatization bubble, lobbies told them everything was fine, and they began to stop to maintain and to phase out public nuclear power plants.
And now we no longer have electricity.
The backup would be electricity made with natural gas. But you need to save that natural gas now. Russia froze the export of natural gas. And other companies know that gas is in short supply, so prices skyrocket.
So yeah, we have to turn everything off at night.