Did FL Studio just crash before you were able to save the project? No worries. If you have autosave turned on, you should be able to find at least a few backups in the back up data folder.
First, check the browser inside FL Studio. You should see a folder called "Backup" in light green color. It looks like this:
As you can see, there has been a lot of autosaved versions of my projects.
Can't find your project here? Okay, let's try this instead.
Go to your backup folder in the file explorer. You should be able to find it in this location: