WiFi self assigned IP and no internet connection after OS & iTunes upgrade

Question

I have two likely related issues that started as a direct result of upgrading to OS X 10.8.5 and/or iTunes 11:
Starting tonight, after upgrading my OS and iTunes I am no longer able to use Home Sharing between my iMac and Apple TV. Apple TV is logged in to my network and Home Sharing is on, on both devices (signed in to the same account), but Apple TV says Home Sharing is not connecting to my iMac. My iPhone 5 is also not showing a Home Sharing connection.
I've restarted both my iMac and Apple TV several times. Logged in and out several times. Turned Home Sharing off/on several times, power cycled and reset my network modem once.
While trouble shooting my Home Sharing connection I noticed that under my Network settings, Wi-Fi is yellow instead of green and reads "Self-Assigned IP... Wi-Fi has the self-assigned IP address xxx.xxx.xxx.xx and will not be able to connect to the Internet.".
When I go back to Apple TV I've found that althought it's connectted to my iMac through WiFi and I can see my purchased movies I can't view them becasue there is no connection to the iTunes Store (no internet coinnection through WiFi). But my actual network connection on my iMac is just fine.
So it seems that my new OS or iTunes upgrade has caused my WiFi to no longer allow an internet connection across teh wireless network.
Can anybody here tell me why that would happen and how can I fix it?
Any help is greatly appreciated,
David

Answer 1

Back up all data before making any changes.
Step 1
Take all the applicable steps in this support article.  
Step 2
If you're running OS X 10.8.4 or later, run Wireless Diagnostics and take the remedial steps suggested in the summary that appears, if any. The program also generates a large file of information about your system, which would be used by Apple Engineering in case of a support incident. Don't post the contents here.
Step 3
If you're not using a wireless keyboard or trackpad, disable Bluetooth by selecting Turn Bluetooth Off from the menu with the Bluetooth icon. If you don't have that menu, open the Bluetooth preference pane in System Preferences and check the box marked Show Bluetooth in menu bar. Test. Continue if you find that Wi-Fi is faster with Bluetooth disabled.
From that same menu, select Open Bluetooth Preferences. If the box labeled Discoverable is checked, uncheck it. Click the Advanced button, and in the sheet that opens, uncheck the top three boxes, if any are checked. Click OK. Enable Bluetooth and test again.
If the application called "Bluetooth Setup Assistant" is running, quit it.
Step 4
This step will erase all your settings in the Network preference pane. Make a note of them before you begin, and recreate them afterwards. It may be helpful to take screenshots of the preference pane.
Triple-click the line below on this page to select it:
/Library/Preferences/SystemConfiguration
Right-click or control-click the highlighted line and select
Services ▹ Reveal
from the contextual menu.* A folder should open with an item named "SystemConfiguration" selected. Move the selected item to the Trash. You may be prompted for your administrator password.
*If you don't see the contextual menu item, copy the selected text to the Clipboard (command-C). In the Finder, select
Go ▹ Go to Folder...
from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.
Step 5
Reset the System Management Controller.
Step 6
Make a "Genius" appointment at an Apple Store, or go to another authorized service center.
Back up all data on the internal drive(s) before you hand over your computer to anyone. There are ways to back up a computer that isn't fully functional — ask if you need guidance.
If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to an empty drive from any of them.) Don’t erase the recovery partition, if present. 
Keeping your confidential data secure during hardware repair
*An SSD doesn't need to be zeroed.