If back-end is not working, what pages does front-end show?

I'm making some pages with React and spring boot.

But, i had one question while making this page.

If some unknown problems occured, server send 500 Internal Server Error or nothing.

In this case, what the pages does React show?

Just show block page? like "oops... there's something wrong...." or show alert modal or something else?

If you advice to me, it'll be very helpful for me.

Thank you for reading this question.