Screen sharing suddenly stopped working
Go to System Preferences -> Security &amp; Privacy -> Firewall -> Firewall Options and remove anything in the list box lower section that has to do with screen sharing. You can remove an item by highlighting it then clicking the minus button below the list. Click Ok.

Next go to System Preferences -> Sharing and uncheck Screen Sharing.

Reboot. I don't know why this is needed and I hate that it is, but it didn't work for me to do this without a reboot.

Then go back to System Preferences -> Sharing and recheck Screen Sharing.