I was supposed to fly home from Santiago de Compostella when the blackout happened. Me and my girlfriend had checked out of our hotel and headed to the bus stop to take the bus to the airport. The blackout had already started but we didn't realise (in hindsight, I do remember the pedestrian crossing not working. But I didn't think much of it). Anyways our flight was cancelled and it was clear we needed somewhere to stay the night.
I immediately rebooked the same hotel, but when we got back there the receptionist had left so you had to check in over the phone instead. Except WhatsApp wasn't working. Then mobile data went down. And before long we were walking through the old town going hostel to hostel looking for a place to sleep, as everything got darker and darker (due to the lack of powered street lighting). The old town in almost pitch black was pretty scary!
We ended up breaking back into the hotel, borrowing a bunch of towels from a laundry cart in the hallway and sleeping in this lockable room we found in the basement.
Besides that somewhat stressful part, it was a really strange but fun experience to see the city without power: no traffic lights, darkened shops with lots of phone lights, cafés still operating just with only outdoor seating and limited menus, the occasional loud generator, and most of all the people seemingly having a great time in spite of it.
I would've loved to have stayed out all night exploring the city, but finding somewhere to sleep that night was a bit more pressing!