If you were asked to choose whether to upload/download, then you may have overwritten your changes. If not, then it may be a bug in AnkiDroid. If you can cause it to happen again in the future, please report the issue to the AnkiDroid developers:
After some further googling around the time I reported this issue I realised that certain cards are hidden when they are new or something like that? I don't remember the details - do you know what I am talking about?