Sounds like the classic case of a corrupt project.
Go to your back ups. No back ups? Go to your Auto Save folder.
No usable Auto Saves? Go to your Creative Cloud folder and look for an Auto Save there.
If you have nothing to work with:
In the future, may I suggest a more robust back up plan for your project files - especially those files that grow larger and larger and get saved across version over version.
Most people back up and duplicate in 3 places at least once per day. That has saved me over the years.
If you have large projects that are inherently subject to the rigors of being large and complex, look into the new feature called "Productions" where portions of your project are broken up into smaller chunks.
There is one last option to try and that is to create a brand new project and import that legacy project into the new one. That can often "rewire" your project file well enough to open it and continue working.
If I am remiss in diagnosing your issue, I apologize. This is usually the case with the symptoms you've described.
Let me know if any of these suggestions worked. Hope to help you!