I recently added a package to my Laravel 4 site and now anything that uses Eloquent (or at least Eloquent with any reference to date/time) is showing a 500 error that states:
Class 'Carbon\Carbon' Not Found.
I tried running
composer install
composer update
composer dump-autoload
Not saying this is work for you, but those are steps that usually fix Laravel, when the problem is not on your source code, of course:
cd /your/application/dir
rm bootstrap/compiled.php
rm -rf vendor
composer install --no-dev