Had a bit of a panic this morning when I let my MacBook go to sleep, and upon re-awaking the screen’s backlight would not come on. The screen itself was working, as I could see a small portion of the desktop in the middle where the Apple logo is, but the rest was un-viewable.
It’s working now, but not after trying almost every trick in the book. I’ve listed all of the methods here, mostly for my own reference in case it happens again but also for anyone having a similar problem.

Diagnostics

First of all, make sure that the rest of the computer is working. If you have an external display handy, plug that in and use Fn+F7 to activate it (you may not need to hold down the Fn key depending on your Mac’s settings). If you can see your desktop on the external display then the problem is with the display – if not, you computer may have deeper problems. If you have VNC set up, you could try this too.

1. Set your brightness

Sometimes it’s the basic things – your brightness might be set to minimum. Use Fn+F2 to turn up the brightness (you may not need to hold down the Fn key depending on your Mac’s settings).

2. Zap your PRAM

One trick to deal with minor hardware problems is to reset your PRAM and NVRAM. To do this, power down your Mac, then, turn it on and hold down Command+Option+P+R at the same time, until your Mac makes the ‘ding’ noise again. Command is the Apple key, Option may be labelled as ‘alt’ on your computer.

3. Reset your Energy Saver settings

You may need to use an external monitor or VNC for this. While you Mac is booted, open System Preferences and choose Energy Saver. Under ‘Put the displays to sleep’, set the timer to 1 minute, and then wait 1 minute for your machine to go to sleep. Wake it up, and hopefully your screen will spring back to life.

4. Take out the battery

If your Mac is a portable, turn the computer off, unplug the AC power cord, and take the battery out. Next, hold down the power button for 5 seconds, and the re-insert the battery. Then boot up again. In my case, this is what worked.