Sooner or later it had to happen. The ransomware blight has been seen on Mac OS X.
Don't panic, don't pay up. The authors are counting on Mac users being less aware of this scam than their Windows counterparts. Not that Windows users are particularly prepared for it when it happens ...
Getting that blocking page out of the way though is a bit of a problem. If you're running Safari it seems that selecting 'Reset Safari' will do the trick. If you've got another browser it may be a little more tricky.
Infoworld have a brief report, but there's more detail (and a how-to-remove video) in an article from Malwarebytes.