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
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 the most important thing is if you are using the anaconda command prompt it may cause a problem. Use windows command prompt instead.