This post details how I fixed a problem with my laptop where attempting to hibernate the computer resulted in it locking instead.
This seems to be a reasonably uncommon problem; there are results on Google but there is no clear one-size-fits-all solution. This is largely due to there being multiple reasons this can occur. Whilst you may be more likely to see this problem if you dual boot Windows with another OS, I started seeing the issue after upgrading my laptop's hard drive.
Every day, when finished with the laptop, I click the hibernate button, close the lid and place the laptop away in a drawer. Since upgrading my hard drive however, I've found that the next morning the laptop's battery is completely drained and prompts me like I've let the computer running. Tracking that down today, I discovered that when the hibernate button is pressed now, the screen goes blank for a few seconds before returning me to the lock screen. Still fully powered on, just locked.