This happens with many of us. Out of two partners, one takes a unilateral decision to break away and does not inform the other. There may be many reasons for going cold, including finding some body else. But is it worthwhile chasing the cold date any more?
No. But most of the break away partners don’t say so clearly and the change in relationship is not spelled out. That causes problem for a certain time. This is surely unethical. One should never behave like this. But you can do nothing about it.
You have no option but to wait for some time and then ask straight question. Are we separate now? Are you no longer interested in meeting me? After getting a clear yes, begin again. Don’t ponder and worry over why the earlier person behaved so strangely. Some people are like that. Imagine, if you had got married to somebody so unethical and unreliable?
This kind of behavior certainly shocks. But there is no use. Try to recover as soon as you can. This time, be more careful selecting your new dating partner and hope for the best. Despite many failures, success always comes. That is your right. Please claim it.