The PSD file got corrupted somehow. Lacking a backup file, your only option left is to attempt to open the file in another PSD compatible application to see if at least some of your work can be rescued.
I have used Photoline (www.pl32.com) before to save lost layers of work in corrupted PSD files before. The trial version is fully functional, so try that, and report back if it worked out for you. With some luck you ought to be able to retrieve at least some of your layers.
And next time use versioning. With each major change, save a new version.
Also, a backup temp file might still be available on your system - but by now, it is probably gone.
System drive (C:)/Users/ your user name here/AppData/Roaming/Adobe Photoshop (CS6 or CC)/AutoRecover
I really feel for you, but we see a good few posts like your in an average year, and they never have a happy ending. I am not even sure how your particular issue could have happened. A fairly common Photoshop gotcha, is after saving to PDF with layers turned off in the save dialog window, and later saving the PSD with layers still turned off. That one has got me twice with a days work lost.
This is not going to help you, but because we see this so often, regular posters on this forum tend to save large projects with incremental file names every hour or so. It only takes a system crash part way through saving a PSD file to wipe it out. My favourite thread was a young lad doing a school project on a laptop using battery power. He could see the battery running low, but tried to finish the job anyway until things looked desperate. He then started saving the file and ran to find the power block, but the laptop died part way through the save. He lost the whole project.
So I wish you luck recreating your project.