What To Do When an Ex Comes Back?

You broke up. Years passed, and both of you moved on. Occasionally a reminder pops up of the time spent together, but for the most part there's no contact.
Then, one day out of the blue, the ex says hi. Maybe you ran into each other on the street, he found you on Facebook, she sent you an email. They ask, "How are you doing?" and, "Maybe we could catch up sometime."
For many, fear sets in. Do I want to see this person? What could they possibly want? Is a reconciliation possible? Are they getting married? Do I really want to bring all those old feelings, memories, back up again?
It's a tough decision to meet with an ex, depending on how the relationship ended and how much time has passed since its demise. I've heard all sorts of stories:

• High school sweethearts that haven't seen each other in 30 years, who meet again at a reunion and by the time dessert is served have already chosen their wedding date;
• A couple breaks up because one has to move far away, just to find themselves next door neighbors in a completely different town four years later;
• Two people share one great date but circumstances don't allow for another. Seven years later, one contacts the other on Facebook, asking for meet up number two.

When readers and clients ask me if they should either contact an ex or reply to contact from an ex, my answer depends on the nature of the relationship and how much time has elapsed. But the stories above prove that sometimes, the rules aren't worth following and connection has its own time line. So what do you think? If an ex contacted you, would you reply? Is there an ex that you want to make contact with again? If you've had an ex contact you, or if you've already done the contacting, how did it go?