There are quite a few. Take your pick until you find one that works.
Change your router channel. Sometimes this is all you will have to do.
Power cycling the router. Read the router's user manual or contact their tech support for instructions.
System Preferences/Internet & Network/Network
Unlock the padlock
Locations: Automatic
Highlight Airport
Click the Assist Me button
In the popup window click the Diagnostic button.
System Preferences/Network- Unlock padlock. Highlight Airport. Network Name-select your name. Click on the Advanced button. Airport/Preferred Networks-delete all that is not your network.
Place a check mark next to "Remember networks this computer has joined." Click the OK button and lock the padlock. Restart your computer.
http://support.apple.com/kb/TS1920 Mac OS: How to release and renew a DHCP lease
No internet connection (wireless)
Check to see if an extra entry is present in the DNS Tab for your wireless connection (System Preferences/Network/Airport/Advanced/DNS).
Delete all extra entries that you find.
Place a check mark next to "Remember networks this computer has joined."
Other resources to check into:
Troubleshooting Wi-Fi issues in OS X Lion and Mac OS X v10.6
Netspot
How to diagnose and resolve Wi-Fi slow-downs
Pv6 troubleshooting
Mac OS X 10.6 Help: Solving problems with connecting to the Internet
What Affects Wireless Internet?
Solutions for connecting to the Internet, setting up a small network, and troubleshooting
