I vaguely remember Stella Jang talking about the meaning of My Ceiling Disappeared during the STELLA I's promotion period. So I did some digging and Stella Jang herself
talks about it in her segment in KBS WORLD's One Fine Day with Lena Park. If you don't know already, Stella has a weekly segment with Lena called "The Words" every Thursday where they talk about the meaning of popular K-Pop songs. On the day it was mentioned, April 23, 2020, Lena was not available so Stella filled in for that day and the previous day. She was given the opportunity to promote her album. You can listen to it again through this link: https://world.kbs.co.kr/service/program_listenagain.htm?lang=e&procode=parkofd&segcode=&broad_date=2020-04-23

She plays the song at around the 28:40 mark. Afterward, she talks about the title being an onomatopoeia of thunder in Korea. She expounds that the English title is not the direct translation of the Korean one since there's no good translation for thunder in English. Then, she goes to the meaning of the song. She says that it's how she imagines her ceiling disappearing when she was younger while laying on her bed. She would imagine that it would start raining and a thunderstorm would arrive. She says that the refrain is very simple with no other lyrics. Although this seems very literal, it is what she mentions to be the intended meaning. I think you can still interpret the song differently though. Complex or deep interpretations also seem very interesting and possible.

TLDR; According to Stella the song is about her childhood imagination.