General File Recovery
If you stop using the disk it's possible to recover deleted files that have not been overwritten by using recovery software such as MAC Data Recovery, Data Rescue II, File Salvage or TechTool Pro. Each of the preceding come on a bootable CD to enable usage without risk of writing more data to the disk. Two free alternatives are Disk Drill and Cisdem DataRecovery. Recovery software usually provides trial versions that enable you to determine if the software would help before actually paying for it. Beyond this or if the drive has completely failed, then you would need to send the drive to a recovery service which is very expensive.
The longer the hard drive remains in use and data are written to it, the greater the risk your deleted files will be overwritten.
Also, visit The XLab FAQs and read the FAQ on Data Recovery.