Are you able to toggle the Wifi on and off? can you see your Wifi network? does it accept the password?
Try turning the Wifi off, either in the Control Center (swipe up from the bottom of the screen) or in Settings, Wifi. Reboot the device then toggle Wifi back on and try joining the network. If that doesn't work try turning off your iPhone, then the wifi router. Power the router back on and wait until it is completely ready to go. Turn the iPhone on and try to connect. Are you able to connect from a computer?