I had this happen to me twice and the solution for me was to delete the wifi network and keychain password.
The first time i fixed it by restoring from Time Machine which took a long time. I also tried the renew DHCP, delete com.alf.plist etc to no avail. The second time I found a better solution from someone on this forum:
Go to Network Settings
Turn Airport Off
Click Advanced tab
Select my wifi network and hit minus sign to delete
Click Okay
Click apply
Go to Keychain access in Applications/Utilities
Find your network name in the list and delete that password login
Then go back to Network settings and turn airport back on
Click Advanced
Click plus sign then Show Networks
Choose your network and re-enter your password
Click Okay
Click Apply
Then airport should work again, this is what fixed it for me.
All the best