Macbook pro, built-in screen and keyboard shutting down

I have a MacBook Pro mid 2014 running High Sierra which has started doing this weird thing.

Basically while you're using it, it will suddenly shut the built-in screen (it goes black) and the keyboard and track pad become unresponsive. At first I thought that the whole computer was shutting down but then I realised that if you plug in an external display, keyboard and mouse it is working.

So far, after a while, the screen etc becomes responsive again but the problem is happening more and more often. When it happens, unless you have an external keyboard, mouse and screen handy, it is impossible to restart or shutdown the machine.