The first thing you need to do in unmount the drive so there are no more writing done onto that drive.  If you use the ext2 file system, you can do a web search for "linux undelete" and find tools that can help you.  If you're on an ext3 partition, I'm afraid things look pretty doubtful.  The reason for this is that ext2 marks the block pointer as unused but ext3 overwrites the block pointer with zeros.  This does not mean it is impossible however.  Here is something that may give you a glimmer of hope.    http://www.xs4all.nl/~carlo17/howto/undelete_ext3.html

I cannot testify to this article but I hope it works for your sake.  I know what you're feeling.

 

my system is FC7. Now it works strangely. For example, I can't open a terminal. I don't know how I can check what file system I had now. Is a workaround when I don't have a terminal?

This is my "mount | column -t" result. It seems it was ext3

[root@localhost stevenh]# mount | column -t
/dev/mapper/VolGroup00-LogVol00  on  /                         type  ext3         (rw)
proc                             on  /proc                     type  proc         (rw)
sysfs                            on  /sys                      type  sysfs        (rw)
devpts                           on  /dev/pts                  type  devpts       (rw,gid=5,mode=620)
/dev/sda2                        on  /boot                     type  ext3         (rw)
tmpfs                            on  /dev/shm                  type  tmpfs        (rw)
none                             on  /proc/sys/fs/binfmt_misc  type  binfmt_misc  (rw)
sunrpc                           on  /var/lib/nfs/rpc_pipefs   type  rpc_pipefs   (rw)
 

Yeah, I think the switched a while back.  In fact I think it was when fedora was still red hat.  Anyway I was doing some thinking after I posted that last reply and I think the glimmer of hope just went out.  I forgot that you mentioned that it happened during an install of another package.  This may have over written those blocks.  Well don't give up, I'm still keeping my fingers crossed.