Mac can't connect to wifi network after password change
If you have checked both the user keychain and the system keychain for old passwords, we're left with obscure guesses.

Two things come immediately to mind (from the obscure side). 


Your mac will join any base station with that SSID. I have seen people literally setting up a second router in busy coffee shops - you have to hold the option key and make note of the mac address you have connected to and see it change to catch this.
If your wifi connects to the router wifi, it doesn't mean the dhcp server has any more addresses to lease. You will need the owner to reset or power cycle the router to see if the leases are all out and unexpired. You basicall have a great pipe connecting you to the main, but no water is flowing to you until you get a lease.