Welcome to MobileRead!
Do you have a backup copy of your library? Calibre doesn't create one automatically, so if you deleted your books without a backup, unfortunately you'll have to add them back manually. Not necessarily one by one, though - there are options to add books in bulk, which you can see by right-clicking on Add Books. If you want to preserve the existing metadata, then it'll be more complicated - you might have to add formats to the existing book records one by one, or merge newly added books with older entries manually.
And of course, always, always keep a backup. Just a copy of your library folder on an external drive is enough.
You may want to ask for more advice in the calibre subforum here:
https://www.mobileread.com/forums//f...play.php?f=236