Hmm, yes we've been having the same problem with our DD. It's very upsetting. This is what I think - and I've given it a lot of thought over the last little while!
Firstly I would go in and have a proper chat with the teacher. Try to find out exactly what is going on. The teacher should be able to tell you what the problems are. It's really difficult for us as parents to know, because we're not there. I think there are two main things that are important to find out.
Firstly: how your DD is behaving. For example, does she initiate with others and what happens when she initiates? Do other children initiate contact with her? How does she respond. What is she like when she plays with others? Can she share, take turns, negotiate? Does she always want to lead etc? Is she doing anything obviously wrong that annoys the others? Is she just not included or is she actively rejected? You may be able to identify particular problem areas that you or the school could work on e.g. initiating successfully.
However, I think teachers can over-emphasise the importance of individual difficulties and social problems always need to be seen, and dealt with, within a social context, so secondly, you need to find out about the social classroom environment. For example, are the dynamics of the group such that few children in her class would be willing to be her friends? This couldl happen if the groups really tightly formed, if your child has very different interest to most of the others of if there is a particular dominant child who may have have taken a dislike to your DD. Also, I think it would be important to find out what kind of work the teacher has been doing with the class around friendships. How often do they explicitly talk about friendships and social skills in the class. What steps do they take to promote friendly behaviour in the class etc.
I think discussing the environment is really important because any social problems should be addressed by involving others. For example, a child may be reluctant to initiate contact with others because they may have learned that when they do, they are met with rejection. There is absolutely no point simply encouraging this child to initiate more because they are likely to get the same response. The teacher would have to engineer a situation involving other children so that when the child asks to join in, the child's is accepted by the others. This can be done in lots of ways e.g. talking to the whole class about what it feels like to not be allowed to play, making all children play with someone new for ten mins that breaktime, giving your child jobs to do with very friendly child so they can get to know each other better etc.
By the way, I think it's massively unlikely that your PND has anything much to do with this. Please don't beat yourself up about it. Get over to the school and try to get them to come up with a plan of action.