No, you can
NOT recover data at this stage if you erased the drive (can you tell us exactly what you did to "delete" it?)
Continuing to use the computer can result in an increasingly lesser chance of recovering the files & a higher recovery bill.
If this project is important and financially valuable, I suggest you
stop using the computer immediately and seek professional data recovery services (that is, you remove [or have someone remove] the hard drive from your iMac, send the hard drive to a data recovery firm, and the firm recovers the data and sends it to you on a separate external hard drive.) The more you use the system, the less chance of success and more expensive the recovery can be. You could be looking at anywhere from $200 to $4,000, depending on what has happened. If this is a high-dollar project, this is a justifiable expense. As DeltaMac noted, your hard drive may be failing or have failed - given it's age is at the point where failure rates increase and all of these issues, it is probably wise to replace it. If this is the case, you again need professional data recovery services. You will also need to replace the failing hard drive with a new one.
Because we still are not clear on exactly what you did in erasing the drive, I highly recommend having a professional do this work if the files are important.
Here is a Proposed Plan-Of-Action:
- Determine what actually happened -- When you say you 'deleted' the hard drive, what exactly did you do? (did you erase it in Disk Utility? - tell us what you did step-by-step)
- Determining if the hard drive has failed -- you can take the system to an Apple Authorized Service Provider who can perform SMART diagnostics on the drive and determine if this is a case of accidental deletion, or accidental deletion combined with physical disk failure. They will also give you a spot analysis on your options for recovery.
- Determining if anything else is wrong with the iMac -- the AASP can run a diagnostic on the machine to check for other causes of failure
- Data Recovery -- removing the drive from the iMac, sending it out for recovery, and getting your files back
- Fixing the iMac -- replacing the hard drive with a new drive, reinstall OS X
- Implementing a proper backup solution -- most likely, this is either going to cost a lot of money to recover your files, or you will have to accept the data loss (and income/productivity loss that comes with it.) A good backup solution will ensure you never have to deal with this again, such as using Time Machine or Carbon Copy Cloner with an external disk drive. One copy of your important files is a huge gamble. Two copies of your important files are good. Three copies are golden, and provide wider spectrum coverage against the many different causes of data loss.