Besides the d folder there should be a Masterkey.Cryptomator file.
It is created during vault creation and stored in the root of your vault folder where also the d folder is stored.
It it is not there then you can recreate it with your recovery key you (hopefully) have noted down during vault creation.
Read here how this is done: Recover masterkey file with recovery key? - #4 by tobihagemann
You can also check you recycle bin for the Masterkey file. Some process obviously has deleted it if it’s not there.
Then you moved the files into the vault instead of copying them. Then of course the source is not available anymore