Thanks, Kovid. After a second reboot, it launched with a message that my database had been corrupted, and attempted to execute a rebuild, which failed. It prompted me to create a new library. My original library is in a Dropbox folder, and it appears that everything is "there." Now, when Calibre launches, I get the blank, new library. I'm barely literate in database use. Any idea on whether the original library is salvageable?
It's Working now - Within Calibre, I went to the "Switch Library" option and selected the original library from the list. It opened just fine.
