iPhone won't connect to some Wi-Fi hotspots
I'd go into settings and erase all the stored SSID/passwords by choosing:

General > Reste > Reset Network Settings

At that point, you would need to try joining the network and be prepared to gather more information about that location. Do other iPhone work with the router, can the router be rebooted (or checked to be sure that there are DHCP leases available - many routers issue only a fixed number of valid IP addresses).

You can troubleshoot things quite well following Apple's guidance too:


iOS: Troubleshooting Wi-Fi networks and connections