Leads disappeared but appearing in search

Hi guys,
Something strange has just happened in our v13 Lead List. Without any user interaction, the Leads simply disappeared, all of them.

However, you can still find them linked in Contacts and through the global search. When clicking on the Lead in the search results, the error page appears “Sorry! I could not find what you were looking for.”

Any idea what could cause this?

I checked the Activity log, there was no user activity in there.

The error logs also don’t show anything.

Your assistance will be greatly appreciated.

Julian