Hi, this is more natural: "Dear XXXX (manager's name, e.g. 'Mark' or 'Mr Roy'), I would just like to inform you that we have been working on this project for a very long time. We have devoted a lot of time and effort to it, but we have just been told that it has been cancelled at short notice. This news has left us feeling quite disappointed and frustrated. This is a very critical project and it means a lot to us. If we had continued working on it, it would have been very successful. I would therefore like to ask Sally to have a discussion with our team in order to explain the situation in more detail. Best regards, XXXX (your name)"

Subject: Project cancellation Dear [Manager's name], I am writing to express my disappointment at the cancellation of the [project name] project. I know that many of us have devoted a great deal of time and effort to this project, and we were all very excited about its potential. I understand that the decision to cancel the project was not made lightly, and I appreciate your transparency in communicating this decision to us. However, I believe that this project is still very critical to our company's success, and I am confident that it would have been a great success if we had been able to continue working on it. I would like to ask if you would be willing to meet with Sally and our team to discuss the possibility of reviving this project. We believe that we have a strong team with a lot of valuable insights, and we are confident that we can make this project a success if we are given the opportunity to continue working on it. Thank you for your time and consideration.