You've deleted a file you absolutely have to have. What to do? Where to start? Well, unless you have a time machine, you need to stop using the drive immediately, and attempt to undelete the file right away with file recovery software.
Any further activity of any kind, using the drive with your files on it, could overwrite your files with data and render them unrecoverable. You should quit all your applications, save nothing, and attempt to recover your files from another disk altogether, like a USB thumb drive or even a floppy:
Download file recovery software to another disk. An easy, portable, and free piece of software to use is Recover Files 2. You can use it directly from another disk, even a floppy. Start the program, choose the disk where your files you need were, and choose 'Analyze'.
Find the file you need in the list of recoverable files. The analyze operation may take a while, but when it finishes, a list of candidate files will appear. Each file will also have a prognosis for undeletion next to it as well. 'Very Good' is the best, usually, and 'Overwritten' means that some part of the file has already been overwritten. That does not mean you can't get some portion of the file, though, so don't give up hope completely.
Choose your files to recover, and choose 'Undelete' (and cross your fingers). Chances are, if you saw the file in the list, some portion of it will be recovered. Give them a shot, and hopefully things will work out for you. In general, the less you have used your drive since the accidental deletion, the better you the chance for undeletion is.
We hope this helps you if you have this problem, let us know if it helps.