I accidentally deleted the users from the C drive and put it on the D drive. How can I fix this so my system works again?

I was low in space in C drive, I copied users from C drive and pasted it to D drive and deleted the users in C drive.

I realized I have made big mistake by doing this. How can I undo the damage and restore the C drive to normal?

You should copy the files back. Keep in mind that the users folder is a special folder. If the permissions are not set correctly, you will not be able to boot from those profiles, resulting in creating a TEMP folder inside C:\Users.

Because the special rights on this folder, creating a symlink will not work. You have to copy the C:\Users folder back first and hope that the rights have copied back with it. If not, your own user must be the owner of your folder and have full control on the folder too.

C:\Users, System and Administrators both have full control on the file, Users is read only. SYSTEM user is owner of the folder C:\Users.

After setting all permissions as it should be, reboot and pray.

Once this works again by setting the folder rights as above, you can then move folders from inside your user to another place and use mklink /d <name> <target> from an elevated command prompt to move folders. Although this will work with appdata, documents, desktop, etc... it will not work on your user folder itself.