Some things to verify:
Try running
php artisan serve
to start the web server only for development purposes. You should have access to
localhost:8000
If it does not display Laravel's splash page, then there can be a problem with Composer. If it display the splash page, it could be
- Apache configuration
- app/storage directory not being writable by PHP
Also remember to check the Laravel logs for errors in addition to Apache error logs (Two last suggestions by @fideloper in question comments)