One morning last December around 4:00am, I wake up to a piercing, shrill, alarm-like noise. In a panic I jump out of bed thinking it might be a phone or my car alarm or something but as I was trying to locate the origin of the noise, I realised it didn't change frequency or volume regardless of where I was in the house - there was no point of origin (as far as I could tell with my semi-conscious brain). So I'm stumbling around in complete darkness with a tiny flash light when I think to myself 'how stupid am I?! I just did the cliche horror film thing and started walking around in the dark instead of turning on the lights' - I find the light switch...click...click...click! CLICK! CLICK! It doesn't work!! I look outside, the whole neighbourhood's pitch black, no street lights, no cars, nothing. I grab the phone, it's dead. I find my mobile and call a friend, they don't pick up (probably because it was 4 in the fucking morning!! However my twisted brain led me to the conclusion that something awful had happened) So there I am, the shrill noise is still there in the background, I'm standing in complete darkness, and perhaps the sleep deprivation combined with the rush of adrenaline caused by my immense fear led me to the notion that the end of the world had arrived. I thought, whatever's happening doesn't matter, the adrenaline was wearing off and I felt exhausted and pretty down in general so I slinked off back to bed, falling asleep thinking I'd rather die peacefully in my sleep than fully aware of imminent death in the darkness.
That day, I found out our power had been cut in the night which set off a neighbour's home security system (he wasn't home to turn it off) which explains the lack of electricity and the awful noise.
TL;DR - Neighbour's alarm led me to believe the apocalypse had arrived.