after a gap of 2+ years: is this terrible idea or not?

I bumped into an ex-girlfriend purely by chance yesterday afternoon in Piccadilly station. We'd gone out with each other for a few months in 2005 but for one reason or another it hadn't worked out, but we still styaed pretty good friends. She moved to New York and got engaged and I assumed I'd never see her again.

Then I bumped into her yesterday and she's split up with him and back living in Manchester. It was totally surreal, like a hologram of her was there talking to me or something. She had to catch a train but gave me her number and we've been texting lots and are apparently meeting up after this weekend.

So I'm wondering if its a bad idea to resurrect past relationships, has anyone else ever done this and it turned out ok? Or has it been an awful idea and in retrospect you wish you'd never bothered because you'd both moved on?

I am massively confused about the whole situation at the moment.