How do I "unlock" a "locked-by-theme" site?

Hi, we have a problem regarding one of our installed themes, Avada. When someone signes up this is the default theme. But when trying to change to a different theme, (any theme), the site locks. It goes white. Front end as well as back end. It is not possible to revert this. (well, it probably is, but we don't know how). And that is my question. And what can we do to prevent this from happening?