Be warned, this is not trivial... :-(
For the future I suggest you to have a look at this
http://directory.fsf.org/libtrash.html
It is a wrapper library (LD_PRELOAD hack) around the system calls which Linux uses when deleting files, and which moves them to a configurable location instead.
However, for now, it might be easy but it might be as well impossible.
I once read that you should for heaven's sake not unmount and re-mount the partition (that is, not reboot!).
If there is still any process running which holds the specific file open, then you're lucky and just copy the file descriptor from the /proc directory.
Here's a guide for that:
http://www.linux.com/article.pl?sid=06/10/30/1652211
There are utilities for ext2 based file systems. One is called
recover, and one
e2undel. But you need to remember some informations about the files. And it won't work with ext3...
Also, depending on file type you might want to try forensic utilities. These scan the whole partition for "magic bytes"/headers of known filetypes and *try* to restore them.
* magicrescue
* foremost
*
http://www.cgsecurity.org/wiki/PhotoRec
* probably others... search for 'forensic' in your package manager
good night, and good luck
