You can start with “suddenly, the lights went off…” and continue with a plot. It actually depends on whether the essay should be a descriptive or a narrative essay.
If a descriptive you should:
explain the feeling of staying in the darkness
convey the idea with many different literary devices like metaphors, similes and personification.
If Narrative it should:
Include dialogues.
explain a bit about what it looks like and compare it to a feeling.
Include a beginning, climax and an ending.
maybe include a few characters♀️.
Hope this helped. Lmk if u have any more questions✨