Would be useful to have more details or situation, error messages your users are seeing, any changes made to your environment.
Some thoughts based on details provided:
- Wifi password changed on your APs or AP firmware upgraded
- New policy distributed to machines (you can confirm they have the correct password by looking at Wifi password through KeyChain)
- Someone with access to a shared iCloud account accessed an SSID with same name but different password (saw this in my environment where one of locations decided to use same SSID but different password and this conflicted for the users when they returned to primary office)
