If you're still carrying a torch for your ex and it's been a while (months, or even years), the approach you take is going to be quite different than that of a recent breakup. In some cases, if a breakup is relatively fresh, the couple can reunite and simply pick up where they left off. That's not the case with a long-term breakup.
There are many reasons why you might find yourself in the situation where your ex comes back into your life. Perhaps they broke up with their new partner, moved back into town, or simply appeared out of nowhere one day through a twist of fate.
Whatever the reason for your ex making an appearance, you must remember that the old relationship is long gone and they may not even be the person you remember. If there's going to be any chance of rekindling those old feelings, you're going to have to start from scratch. That means gradually getting to know your ex again and building a relationship slowly.
Start off by contacting them just to say hi. Depending on how that goes, you might ask them if they want to meet you for a coffee. Slowly build a friendship and frequently try to gauge where things are going. The name of the game here is to go slow until you can feel out the other person.
The good news is that if you developed a relationship once, you can certainly do it again. If you don't rush things and remember to respect their space, you may find that you're talking more often on the phone and meeting more frequently for coffee or lunch. Eventually, you may find yourself in a dating situation just like it was the first time. Be yourself and trust in the fact that things are going in the right direction. Don't try to force anything.
It's important that you don't bring up the old relationship or the trouble that led to the breakup. This is critically important. Just sweep that under the carpet and start fresh. If you hold a grudge and you're not able to stop talking about those painful memories, you're going to quickly ruin any chances you have for a new relationship.
You are two different people now and you've got every chance to create a wonderful life.