I\'ve copied and pasted the example code from twitter bootstrap to create a basic modal window in my Rails 3.2 app:
If you're running Bootstrap 4, it may be due to ".fade:not(.show) { opacity: 0 }" in the Bootstrap css and your modal doesn't have class 'show'. And the reason it doesn't have class 'show' may be due to your page not loading jQuery, Popper, and Bootstrap Javascript files.
(The Bootstrap Javascript will add the class 'show' to your dialog automagically.)
Reference: https://getbootstrap.com/docs/4.3/getting-started/introduction/