beer turned out flat: I added yeast.

So... I was very excited about how my Belgian Strong Ale turned out after letting it sit for a number of months. But when I opened one, it was completely flat. Obviously the yeast must have been dead before I added the priming sugar. Anyhow, since I figured there was nothing to lose, I opened all of the bottles and added a tiny pinch of yeast to each, then re-capped them. I just guessed about how much to add. Anyone had success with this? I looked at the bottles today( they're in the garage just in case) and none had exploded... yet.

How long did it sit in bottles? How long in secondary? and lastly, what is the temperature in your garage right now and what temperature were the bottles at when you tried to carbonate them the first time?

Lots of questions, but the answers will help.

It was in the bottles for around 3 months. They were inside the house, which is regularly 70 degrees. Prior to the bottles, it was in the primary fermenter for almost 2 weeks ( I have no secondary fermenter). The garage is around 55-60 degrees. I put them in there because I imagined they might explode. So I wanted to slow the yeast down just a bit.

is it possible that the caps leaked out co2 at all??

No because when I did open them, they did hiss just a tad. Anyhow, either they will turn out great or they'll explode and leave me a mess to hose out. It was a bit concerning that as soon as I put in the yeast, they started foaming up, which makes me think that there will be tons of pressure in those bottles. I will keep you all informed as to how the experiment went!