Where have all my wifi credentials gone?
This might be the case if the update had included an update for the Android WiFi driver, which might have caused incompatibilities with stored configuration data for networks. Try to retrieve this file (you will need to be root):

/data/misc/wifi/wpa_supplicant.conf

This file contains the configuration data for all WiFi networks which had been saved.

If this file is missing or blank, you will have to reconfigure the WiFi manually.