I'm a simple person. Fortunately, here's a simple problem, with a simple solution.

In my shop, for various reasons, we don't use Apple Remote Desktop as a support tool. However, I use it to connect to test machines from my Macbook. As those test machines are in shared offices, I use the Curtain feature to prevent people seeing what I'm up to. It also lets you leave rude amusing messages on your screen for your colleagues, so it's an essential part of any workflow.

On several occasions, I've returned to a test machine to find the screen locked and the machine unresponsive. The screen will be completely blank, with not even the padlock icon present. This can occur if you log out of the remote client whilst still in Curtain mode.

Apple do have a support article for this, but it doesn't extend beyond "Don't do it" and "If you do, restart". If you don't want to do that, you can simply kill the ARD process via ssh.

First, ssh to the box, then identify the PID of the ARD process:

$ ps -ef | grep AppleVNCServer

You should see something like:

/System/Library/CoreServices/RemoteManagement/AppleVNCServer.bundle/Contents/Support/LockScreen.app/Contents/MacOS/LockScreen -session 256 -msgHex 

Look for the PID. Then, simply use it to kill the process:

$ sudo kill -9 PID

Your screen should jump back to life, and you don't have to risk losing your place on all the cat videos you had open by restarting.