What to Do When You Can’t Meet the Project Deadline

Let's say your manager has assigned a project to you. You're already working on a few priorities, but you accept this anyway. Why? Any one of a number of reasons. Maybe you think the project is going to add to your skillsets, or you want your manager to know that you are willing to take on new challenges, or you just can't say no to your manager. Whatever the case, once you've started the project, you realize, you really don't have the time and resources to deliver. So what now?