Macbook Air suddenly doesn't recognize password

I left my Macbook Air on overnight to export a video. When I started up the laptop again, it didn't recognize my password anymore ... Then when I restarted and held down "Command R" to change pw through terminal ... there was no account that could have its pw changed. Then when I restarted again OS didn't recognize username or pw anymore ... What's going on? is this a hack or is there still a way to login ?