The easiest way to get back missing items? Restore your from one of the multiple backups you make of your iTunes library.
The only things you can get from the "Cloud" are items you have purchased from Apple, unless you subscribe to iTunes Match.
Downloading (using iOS or computer) past purchases from the App Store, iBookstore, and iTunes Store - http://support.apple.com/kb/ht2519 - enabled with iTunes 10.3 and newer; not all media formats are available in all countries (see: iTunes in the Cloud, iTunes Match, and iTunes Radio availability by country - http://support.apple.com/kb/HT5085); apps, books (not audiobooks), music, t.v. shows, and movies (some - not all studios have permitted this). Downloading previously purchased movies and TV shows requires iTunes 10.6 or later. Discontinued items not available. For items not included in the iCloud list (e.g., ringtones, audiobooks, alert tones), or locations or computer systems where iCloud is not (yet?) available, you only get one download per fee paid. Apple recommends, "... back up your iTunes library. " (http://support.apple.com/kb/ht2519). There is, however, this document about ringtones: Restore missing tones on your iPhone, iPad, or iPod touch running iOS 8.1.1 - http://support.apple.com/en-us/ht204076
As for your iPhone, your i-device was not designed for unique storage of your media. It is not a backup device and media transfer was planned with you maintaining a master copy of your media on a computer which is itself independently backed up against loss. Media syncing is one way, computer to device, updating the device content to the content on the computer, not updating or restoring content on a computer. The exception is iTunes Store purchases which can be transferred to a computer.
iTunes Store: Transferring purchases from your iOS device or iPod to a computer - http://support.apple.com/kb/HT1848 - only purchases from iTunes Store
For transferring other items from an i-device to a computer you will have to use third party commercial software. See this document by turingtest2: Recovering your iTunes library from your iPod or iOS device - https://discussions.apple.com/docs/DOC-3991
