Screen sharing suddenly stopped working

For years, I've used screen sharing to connect my MacBook to my iMac. It has always worked like a charm. About a month ago, it stopped working. I can still see my iMac in Finder when I'm on the laptop. But when I try to connect, I get the "busy" bar for a minute or two, then a message telling me I needed to enable screen sharing on the iMac.

Screen sharing is enabled; I never changed the setting, and I've double checked that it is still clicked. I can screen share via my MacBook with my husband's iMac across our home network, so I think the problem rests with the iMac, not with my laptop. I have tried sharing via VNC. I have tried logging out of Mobile me on the laptop. I have tried a screen sharing app. Nothing works. Can anyone think of anything I haven't tried?

^{I'm running OS X 10.7.5 on the iMac.}

Go to System Preferences -> Security & 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.

Check the settings for Remote Management under System Preferences -> Sharing -> Remote Management. For whatever reason, I had an update cause the same issue on my 10.7 machine, and Screen Sharing no longer worked anymore. Under Computer Settings, you can give it a password if you like. Checking Remote management should check the Screen Sharing box for you also.