Once you empty the trash, even if the files have not been overwritten yet, retrieving them is difficult. There are some utilities that may be able to accomplish it.
Prosoft Data Rescue is one of them. But don't do another process on that drive until you've downloaded and run it. Also, there are professional drive recovery services like
DriveSavers.
In short, emptying the trash is pretty much final and you can't trust a recovery method to bring it back. If you have a habit of emptying the trash too soon, create a folder called "move to trash" and put items in there. Then, at the end of the day you can go through and trash the ones you no longer need.
The computer ate my homework is a lame excuse 😉
-mj
macjack@gmail.com