Licenses

I've had several users over the past couple of years log into our network. But recently I've received an error for every user I've tried to set up since. The error reads as follows "The system could not log you on. Make sure your User name and domain are correct then type your password again. Letters in passwords must be typed using the correct case. Make sure that Caps Lock is not accidentally on."
I'm not sure where to go from here. Do I need to add more CALs to my server?