Gladtobemom - what version of calibre were you running before you upgraded to 2.3, and what version of Windows are you using ?
It might be useful to install the portable edition of calibre for the version you were using before you installed calibre 2.3 ==>>
Previous calibre releases
It will create a near empty library default in the install directory. Drop the mobi into the book list and see if you're back to a readable 3 column setup.
BR