I left my laptop on sleep for 5 hours, it was working fine before. Then I turned it back on the keyboard backlight was on but the screen stayed black. I even tried plugging it into an external monitor, and no post. Any ideas?
Are you saying that you managed to turn the laptop off but it wouldn't POST when you turned it back on? If you are, and you're moderately experienced, then disconnect the battery, hold in the power button for a few seconds, then try booting it again.
Note that the battery for your model is not easy to get at (which is the norm nowadays) so you'll need to take the entire bottom off the case then disconnect the battery cable that's connected to the motherboard. Again, don't attempt it if you don't have the confidence and experience.