Back from the dead?

So I'm having a pretty bizarre experience and finding it difficult to see people with similar problems online.

2 weeks ago my Macbook inexplicably died. Its 5 years old and its basically been my baby I've never had any problems with it, it was never damaged in any way.

I was typing and let it run down to 0% battery so it switched off. A few days later I went back and decided to finish what I was doing. Plugged the charger in and the light was flickering between faint orange and green and the macbook refused to power up. I borrowed a friends' charger assuming that was the problem but that didn't work either. I googled it and found people had a similar problem and simply disconnecting/re the battery connector solved it! This didn't work for me and so I gave in and brought it to the apple store where I live. After about 5 minutes the tech came back and said it was dead. The motherboard had corrupted and essentially everything was gone to crap. The figure he gave me to fix it was 900+ euro. Considering a new macbook pro isn't much more and this one is 5 years old I decided to mourn its passing and wait/save up for a new one in a few months. He told me the laptop was totally gone and I could consider exchanging it.

ANYWAY I leave it alone for 2 weeks in its case on a shelf trying not to think about it. Today, I decided I'd try plug it in one last time (I don't know why, maybe praying for a miracle)

I plugged it in and the charger lit up, its charging perfectly. Its running fast, all my files are there. All my software is running without problem including some of my heavier music programmes. The only thing different was the clock had reset to a year ago? I'm so confused. Maybe its a fluke and it'll be gone again this evening but the tech at the genius bar said it was gone forever.... never to turn on again?