It sounds like you might be having trouble expressing a deeper emotion or issue that bothers you. Whenever someone freezes up in therapy, it is usually due to fear of talking something that is truly bothering them.
You mentioned in the past week that you have been feeling more down than usual. And you also talked about your boyfriend and your sadness over the ending of your relationship. What you feel about talking in therapy could be connected to your depression.
Also, you may fear what your therapist will say. If someone is ashamed of how they feel, they can fear the reaction of the person they are telling. They don't want to feel worse in case the person says something they don't want to hear, so they tend to hide their feelings instead and pretend to feel better than they really do.
It may help to write out how you feel. Use one piece of paper and jot down any emotions or thoughts that come to mind. Don't try to hold back anything, even if it seems silly. Most likely what you will get is a good starting list of your true feelings. If it helps, imagine that you are not sharing this list with anyone. That may free up any inhibitions you have about sharing your feelings.
You can also look up a feeling list on the internet and circle your emotions. All you need is a few and your therapist can help you explore them from there.
Also, add two incidents that happened in the past week that stand out for you, things that have strong emotion attached to them. That can also be a good starting point.
I hope it goes well,