Site all jumbled now. Can't access dashboard. Have we been hacked?

Hi all, our wordpress site has suddenly started showing thousands jumbled letters in the header and we can no longer access the dashboard. with the same information being displayed.

Does anybody know what might be going on here? I’ve tried to decode the base64 message being displayed but it didn’t reveal any clues 🙁

http://melbournemagicfestival.com/

Any suggestions would be greatly appreciated. Thank you!