Hello evelynacosta,
Thank you for reaching out in Apple Support Communities. The screen recording would need to be stopped by the user in order for it to be saved in the Photos app on your iPhone. It sounds like you may have done that prior to your iPhone powering off, however, do you recall if you saw the message that the video was saved? It would have appeared at the top of the iPhone screen as a banner alert.
Just to be sure, you can check the Recently Deleted album or the Hidden album in the Photos app. Check and see if the screen recording appears there. This step may help you to locate that missing video. However, if the video was not saved, and you are unable to find it in the above locations, we regret that we'd be unable to get that video back for you.
Best regards.