I was trying to symlink Windows desktop with Lubuntu desktop folder, but have in the process removed the Lubuntu folder completely. I know I can't recover the lost files, but how do I fix this now, since the desktop functionality is now somewhat broken? And/or how do I then symlink a folder (which will be a Windows desktop folder) to Lubuntu desktop folder, so that my Lubuntu and Windows OS share the same desktop? Are there any downsides to this?

EDIT: I'm using dual boot. I have already mounted the data Windows partition and can access it in Linux.

Restoring ~/Desktop

I'm assuming you already ran mkdir ~/Desktop to create the actual desktop folder.

As you didn't specify what part of your desktop functionality is now somewhat broken, my best guess is that when you tried to start without a valid desktop folder, your system instead assumed a new default (probably ~).

As per this old post (and a whole lot of others), it should be enough to make sure your ~/.config/user-dirs.dirs has this line somewhere in it:

XDG_DESKTOP_DIR="$HOME/Desktop" 

(You may need to log out and back in/ restart for this to take effect.)

Using the Windows Desktop in Lubuntu

As with everything in the *NIX world, there are sevaral approaches to this that would/ should work.

One of these is to make ~/Desktop a symlink that points to your Windows desktop. However, since this did not work for you, my guess is that whatever sanity-checker makes sure that your Desktop exists gets run before the windows partition gets mounted and thus "corrects" the path. So discard that idea.

Another approach that should work in theory is to simply adjust above config line to point directly to your Windows desktop folder. However, this will likely fail for the same reason as the first approach.

What should work regardless of any sanity-checking pre-mount -a however is to simply use mount --bind to mount the Windows desktop over your Lubuntu one - with the added benefit that it keeps working if the Windows partition is ever missing or damaged, only differing in content:

mount --bind /path/to/windows/desktop $HOME/Desktop

In fstabthis would look like this:

/path/to/windows/desktop /home/username/Desktop none bind

As long as you make sure that this gets run after the Windows partition is already mounted (probably by putting this line further down in fstab), this should transparently provide your windows desktop in Lubuntu without messing with the underlying folder structure.

The first part of @Entropy0's answer, i.e. restoring Desktop, worked, but the second part not quite. I then made it work with the help of https://superuser.com/questions/251537/mount-specific-ntfs-directory-on-linux.

So all I had to do, is put these two lines in /etc/fstab.

/dev/<Windows_partition_name> /media/<some_linux_username>/<new_mount_directory_name> ntfs uid=<some_linux_user_id>,gid=<some_linux_group_id>,umask=0022,nls=utf8,defaults 0 0
/media/<some_linux_username>/<new_mount_directory_name>/Desktop /home/<some_linux_username>/Desktop ntfs-3g rbind,defaults 0 0

Now when I log into the Linux OS, the Desktop is already mapped to the Windows desktop. You can do the same for Documents, Pictures, etc., or you can just symlink them to /media/<some_linux_username>/<new_mount_directory_name>/Documents, etc., which I did.