Hm, when you delete a database from the openLCA UI, also the phyiscal files in the directory are removed. If you can restore these, or have these still, then you can make it "visible" again in openLCA by modifying a databases.json text file that you find on top of the databases folder. Make a backup of this, then open in a text editor. It looks like this:
insert the name of the database you miss, in the same "curly bracket structure", under local databases, then save and restart openLCA.