After six months of inactivity, Control Center automatically deactivates a user's account. (After an additional six months of inactivity, Control Center deletes the user account.) As a Control Center system administrator, you can reactivate users so they're able to sign in again. You can also manually deactivate users if they no longer need Control Center access.
If it's been a while since the user last signed in, they may not remember their password. They can either retrieve their own password or you can reset their password for them.