Most people don’t realize but if you click on Empty Trash, what you really erase is not the files themselves but the hard drive's directory entries for the files that were in the Trash. The space that the files used to occupy is now (from the hard drive's perspective) "free space" that is "available" and can now be overwritten at any time.

However, your information is still there. You just need to know how to find and extract it. Luckily there are tools available for you to do this successfully.

Your greatest chance of recovering a file is to stop using your Mac straight away and try some type of third party recovery software. The probability of recovering the files decreases substantially the longer you use the computer after selecting Empty Trash option.

Now, there are various tools available.

A program such as Stellar Mac Recovery - (read a review and step-by-step instructions on how to recover deleted files from Trash here) - doesn’t recover deleted files from your Mac like most so called "recovery" tools. It won’t ever for example, allow you to write data to the same volume as that being recovered from.

If you were to do this, it would defeat the whole purpose of recovery, when the recovering files may well overwrite data that is in line to be recovered. So, to operate most efficiently and successfully, you should retrieve files to an external device or secondary drive in the computer.

Third party programs such as Stellar Mac Recovery can also help you recover data from corrupt or deleted hard drives or any other storage devices. Mac data recovery programs will also work if you want to recover lost data from Mac volumes HFS & HFS+.

Finally, your last option is to send the hard drive in question to a data recovery service. You should note however that professional data recovery is expensive although this can be a relative thing - it all depends on how much the lost data is worth to you.