If you don't have a CC database backup then your choices are limited.
- If your books are epub then you can use CC's "scan for new books" to repopulate CC's database. This "scan" happens after you point CC at the folder containing the books and then connect as a wireless device. The problem: if the metadata for the books has changed then calibre might not match the books, considering them "not in library". The scan is enabled in CC / Settings / Connecting to calibre / Wireless device.
- If the books are any other format (pdf, mobi, etc) then the best way to recover them is to resend them. A "scan" will generate books with strange names that don't match the calibre library.
Personally, if I knew what books belong on the device and if the number of books isn't in the thousands, then I would resend them using the Wireless Device.