The Saturdays singer explained that she had got back from work late, and had gone into the bathroom of her London home to get ready for bed when the incident happened.
Calling it a "huge shock", King wrote on Instagram: "After coming home late from work, I suddenly collapsed unconscious on the bathroom floor at 4am, hitting my head and face as I fell ... I ended up being rushed to A E."
She took to the social media site to share an update to fans after being absent from the BBC Radio 1 show she co-hosts with Matt Edmondson.
King admitted the scary incident had been a "real wake-up call" that she needed "to take some time to get my strength back, not just for my own health but so I can be the best version of myself for my family too".
The mum-of-two went on to thank her former England cricketer fiance Stuart Broad for finding her and acting so quickly.
"I keep thinking how grateful I am that I wasn't on my own and had Stuart to bring me round after I fell."
King also shared a picture of herself with one of her daughters, as she continued: "Yesterday was my first day doing anything social.
"After what has felt like a very long two weeks it was amazing to get out and spend tome with my girls.
"We even had the surprise of spotting Daddy on a big screen, which made us all smile."
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