Check your python version by going to your command prompt and type:
Make sure the python version you are using is 64bit. If you are using 32bit python version and installed a 64bit binaries for OPENCV it may cause this error. Or you may use the x86 version of OPENCV file .
You can also download OPENCV from https://pypi.org/project/opencv-python/
Or you can uninstall and reinstall python on your system it may work.
Also you can install opencv by using the command on your command prompt:
pip install opencv-python
And finally if you and using the anaconda command prompt it may cause a problem. Use windows command prompt .