Screen is blank (External monitor)

A slight nudge can sometimes disrupt the connection between the cables. Check the cables that connect the monitor and the computer by disconnecting and reconnecting them again.

You may have reduced the brightness of your screen before and forgot to increase it. For instance, if your computer had to do some processing where the screen was not required (good to save energy). 

In that case, just turn the brightness up again. Check out the don't know how to make the screen brighter solutions to learn how to increase it.

Adding or replacing hardware, requires new drivers to be installed, or the already existing ones to be updated. Every now and again, different versions of different drivers might be incompatible, and they could interfere with each other's work. Uninstall all recent drivers and install new ones (or try restoring your computer to a previous System Restore point). Another thing you can do is to update all the drivers or even re-install them.

Check out the don't know how to update drivers tips if you aren't sure about how to update the drivers.

Note: Check out the don't know how to download and install drivers guide to learn how to download and install the official, safest and up-to-date drivers.

Maybe your screen is black because your device is not turned on, or maybe you have accidentally turned it off.

If you are using a computer and it doesn't turn on, check out the solutions to the computer doesn't turn on issue.

The power outlet used by the device could be malfunctioned and that's why you are unable to power it up your device. Try plugging the device in another outlet nearby, or replace the existing one.

If you are not sure how to fix the power outlet, don't put your life at risk and call an experienced electrician.

Your screen might be blank because your entire device didn't start. 

To check if that's the case, plug it into your computer (e.g. via USB), and wait for at least 5 minutes (because the battery might be empty) and if it is not recognized by your computer then it doesn't function properly. 

If this is the case, ask a computer repair guy to repair it.

Maybe the screen is blank because the monitor is turned off. Before you do anything else, ensure the monitor is switched on.

To make sure, turn it OFF and ON again.