Self-Assigned I.P. Address

Open System Preferences, select the Network tab, select WiFi if it isn't already selected, and click on the Advanced button. You'll see a list of all the networks your computer has logged onto. Delete all that aren't important and drag the router you are trying to connect to to the top of the list. (You might wind up deleting them all but try this first.) Shut down the computer. Shut down the router and wait 10 seconds before turning it back on. Wait for about 2 minutes before you turn on the computer to let the router fully connect to the network.

If the above doesn't do it, remove all the network connections and repeat.

When you see an exclamation point in the Wi-Fi menu, first turn Wi-Fi off and back on. Disconnect all USB devices except keyboard and mouse, if applicable. If it still fails to connect, then from the menu bar, select

 ▹ System Preferences... ▹ Network

Click the Assist me button and select Assistant. Follow the prompts. You may get a warning that Wi-Fi is not available, that you're too far from the base station, or that you're using the wrong password.

Assuming that you've ruled out those possibilities, restart the computer and try again. If there's no change, click the lock icon in the lower left corner of the preference pane and authenticate, if necessary. From the Location menu at the top of the window, select Edit Locations. A sheet will drop down. Click the plus-sign button to create a new location. Give it any name you want. In the new location, set up the Wi-Fi service with the same settings you used before. Click Apply and test.

If there's still no connection, and if you have control of the router, compare its settings to Apple's recommendations. Some third-party routers may be incompatible in 802.11b/g/n radio mode. Try setting the mode to 802.11n only.