Oh mama, I could have written this post a year and a half ago. With us, it was a cute book about cows in a wind storm which happened to look like a tornado and got DS questioning. DH and I did our usual of answering his questions simply and truthfully. What tornadoes are, etc. It didn't seem like a big deal however, this benign situation began a whole new trend in bedtime at our house. *sigh*
I do feel to a certain extent that it could have been anything. This book triggered it, but it could have been another, or another. DS has an immense imagination so it was bound to happen at sometime or another and I think 3.5 is a pretty typical time for this. Like you, we had been enjoying nearly a year's worth of happy, easy bedtime after a rough 2 1/2 year start. *sigh*
I had absolutely NO LUCK with the "spray scenario" that has been mentioned but I do know it works for some so it's certainly worth a try. Best of luck!
For us? DS just didn't buy it, for when he would lay there it was his brain that he couldn't control--it had it's thoughts and nothing or nobody could change that, really. Add to that, he happened to have what I think was his first real nightmare at this time. Yep, I could spray all I wanted but then if he had a nightmare, he KNEW the spray didn't work, etc.
I don't have any real advice here. This whole thing actually afforded us that missed opportunity to co-sleep which we were wishing we'd done when DS was a babe. But I admit, I missed those easy bedtimes! DS does now need me to be with him while he falls asleep, but I step out after he's nodded off. He falls asleep in our bed and we transfer him to a small bed/mattress on the floor of our room when we come in to bed.
This all happened in stages mind you. At first, he just needed me to stay with him while he fell asleep, and then he ended up waking up a lot and most nights, ending up in our bed at some point. It was getting exhausting and going on and on, no one sleeping very well and the whole family getting crankier by the day, the week, the month. I finally put a mattress on our bedroom floor and told him if he needed to he could come into our room if he woke up at night, etc. And he did, with rare exception, end up on our floor most nights. And then, after a traumatic bout of illness (rotavirus) last April where we let him sleep in our room from the first of the night, we ended up just taking the bed out of his bedroom, turning it into a playroom and he's still sleeping with us. And, I think we've still got a ways to go on this one, and we're ok with it, mostly. Some nights I still miss being able to say goodnight and get to my "me time" a little earlier, but DS and I did strike a deal that I could read while he falls asleep with my small nightstand light on... and yes, I have been very good about catching up on my reading so there's a plus!
To say nothing of, we are finally getting the sleep we all need. DS rarely wakes up at night now and if he does, we're right there and he falls right back to sleep. If he wakes before we come to bed (rare or phase-ish), he's usually easy to get back to sleep so long as we keep the door open and he can hear us "about the house."
Hope something here helps you, the spray, comisery, what have you. Best of luck to you and your family!