Calendar Page Can’t Be Found

Everytime I click on a calendar event I get a That page can’t be found. Page.
This is not just old events but also if I create a new one.
If I go into all events they’re all still there and can still be edited. If I go to preview changes the same think will happen and will receive a page not found.
I find if I click onto the permalink I also have the same problem.
If I haver over an event in the calendar it can still be viewed.
Any ideas?

Thanks
Paul