If you uninstall K2, your data is gone. So you may want to be extra careful there.

If there's a chance you may have modified core K2 files, simply re-install K2 (install on top of what you have already).

If you wish to start afresh, locate any overrides in your template (if there are any under html/com_k2) and delete entirely.

Removing K2 but keeping the data tables is pointless. Just re-install if you believe you (or someone else) may have edited any core K2 files.

Perhaps the change though lies in the stock template (again, if it's modified).

In any case, it is a matter of broken HTML. Or was actually as I see your site looks just fine on Chrome (macOS).

Here are 3 screenshots of the site's frontpage (which loads content randomly): jmp.sh/2VYtKQ1

If the layout broke at one point, this may be due to some unclosed <p> tag for example in your introtext. In that case, just enable introtext cleanup (in the K2 Settings) and your HTML will never break in listings.