Once a Project has been deleted, we can't recover it for you.
However, there are ways you can avoid this in the future.
Restrict deletion rights
If you are an Administrator or have permission to modify user roles, you can restrict Project deletion rights by assigning your users a role without this permission.
If you have Advanced User Management, you could also create a new custom user role without this permission or modify your existing user roles so that deletion rights are concentrated among a smaller group of users. You can learn how to do this in Creating new User Roles.
Create an archive
Instead of deleting Projects, you can make archiving them part of your process.
You can create a new folder in your library where all old Projects are held and give it a clear name that indicates it is an archive.
Any time a Project needs to be deleted, your colleagues can move it to the Archive folder.
Then, a user with deletion rights can periodically review the folder and perform a final deletion.
This can work well with restricting deletion rights for users as it provides an option for those without deletion rights to remove old or unused Projects from the library.
Lock the Project
You can lock a Project to prevent any further editing to that Project. This includes the ability to delete the Project. The Project can be unlocked at any time by an Administrator or any user with a custom role that has the Lock/Unlock Projects permission.
You can read more about this in Preventing unauthorised editing to your Projects.