Holly Tree Suffering

Hi, new member here and novice gardener but mad keen. I have a problem with my Holly tree, it lost all of it's leaves over the last few months. It has been in a large plastic pot for the past three years and thriving at four feet tall, it even produced its first berry last year. The leaves gradually turned black and fell off and i just noticed recently that some branches have also turned black so I took the bold step of cutting all the black bits off and chopped about a foot off the top and given it a fair trim around the sides. I transplanted it into the ground a couple of weeks ago as I have just replaced all of my hard clay soil with a nice new topsoil/manure mix and it seems to be trying to leaf again.
Have I done the right thing? What should I do next? What do you think caused this? Could it be that brutal winter, under-watering or did it just knacker itself out squeezing that wee berry out?