First priority, obviously, is your project data. You'll want to create a backup ASAP, before doing anything else, and keep it somewhere safe.

Look for error logs. If you can find anything specific, you have a lead you can follow up on.

Barring that, try to reproduce the problem:

• If you can, copy your entire project to another machine and try opening it there
• If you can, try exporting some or all of your assets and then re-importing them in another project
• If you can, try copying your project folder to another location on your machine and opening it from there
• If you can, try copying just your assets folders into a new (empty) project, and loading that one

See if your Unity install can create and run a new, empty project -- you can get into the project selection dialog by holding "ALT" while the program starts.

You may want to try reinstalling Unity.