Did it ever work? If so usually just resolves itself. If it hasn't/doesn't resolve delete the account and add it back. Check settings with provider if none of those work
Try Reset Network Settings. Go to Settings>General>Reset> tap Reset Network Settings. You may need to enter your wifi passcode back in afterwards. Also try a reboot of your wifi modem/router. A poor wifi signal can cause that.
I think this might have been an outage of some sort. I got the same error this afternoon just before 4 pm when I changed some passwords and was waiting for my confirmation email.