I suspect you are using the relatively new Project for the Web application. Could you confirm, please, that you are using the web-based tool whose URL is similar to the following?
If so, if you are the user who created the missing project, launch your web browser and navigate to the Project Home page using the preceding URL. On the Project Home page, the system displays the list of recent projects. On that page, click the Created by Me link. Do you see the missing project listed there? If so, click it to open it.
If I am misunderstanding your question, please let us know and we will try to help you.