Specifically, we're told this by customer service:
"We have received a high number of inquiries about players losing progress in the game and seeing old missions come back. We apologize for the problem and have our developers looking at the problem now.
There's no need to submit a help ticket for this matter, as we plan on correcting this problem on our end for all affected players. We request if you are having this problem to try again in a few hours (or even later today) to see if your progress has been returned. If this problem has occurred for you, please simply click the "vote to fix" button, this will alert us that you were affected."
Essentially, what this means is that you don't need to contact Zynga Customer Support to fix this issue, as supposedly everything will be placed back to normal on Zynga's end without any input from us. That is, the "game knows" where you were when the bug first hit, and will revert you back to the proper state when the bug has been fixed. As for the time table, things aren't progressing as quickly as we would have hoped. As of yesterday morning, the issue was still being worked on by developers, but a proper fix hadn't actually been pushed out.
While you're waiting for the issue to be resolved, I'd suggest simply not playing the game (or doing anything significant, that you would be upset about having to repeat). Keep checking back with the game's official forums for updates as to the progress of the bug fix, and we'll make sure to let you know if anything significant happens between now and then.
Have you been hit by this "lost progress" bug? How far did it push you back? Let us know in the comments.