This application failed to start because no Qt platform plugin could be initialized

For me, it worked by using a opencv-python version prior to 4.2 version that just got released. The new version (4.2.0.32) released on Feb 2, 2020 seems to have caused this breaking change and probably expects to find Qt at a specific location (Users/ directory) as pointed by other answers.

You can try either manually installed from qt.io as suggested and making sure you get a .qt directory under yours Users directory, or you can use version 4.1.2.30, which works like charm without doing anything else.

It works for opencv-contrib-python too.

Try installing

pip3 install opencv-python==4.1.2.30  

For Ubuntu users,

sudo apt-get install qt5-default fixes the issue.

(I'm using OpenCV 4.4)