how to desactivate a theme?

Sudently, without doing anything, all images disappearred from my dashboard (even when I go view the fie of a picture, information appear, but no images can be seen), on my website (no more galleries, no more background fullscreen slideshow) and even on the Edit page visual option, all the usually displayed designed options over the page layout are empty (just a grey square).

It seems the theme is not working anymore.

Is there something to be done to make it work again??

I have purchased Responsive Fullscreen Studio for WordPress.
I’m now trying to reinstall this theme.

Can I remove the installation of the theme?? So I can reinstal it??

Tks for your help.