Now all what was in /opt (KDE for example) is gone! I know: very stupid of me.
No, it isn't gone. It's still on the hard drive under the root of the file system. You've mounted another partition over it, so you can't see it.
To prove my claim, boot a livecd and mount the original partition, but not the new partition. Then, from a console, or gui filemanager, look at /opt. You will see it all there.
If the new harddive partition is listed in /etc/fstab with mount point /opt, delete or comment out the line, save the file, and reboot.