I had this exact same problem with the first lager I brewed. I didn't know that lager wasn't suitable for kegs. I opened the keg and let the lot go flat, re-primed at 1 heaped tsp per pint, bottled it, gave it 2 weeks in the warmth and put it out in the garage. I tried a small sample bottle after a week and it was well on the way. That was a couple of months ago and I had a bottle the other day and it was fine. It'll be a great summer pint if we get any hot weather.
good luck
good luck