If you can login to WLC then you can reset the username and password.
Login to WLC.
Click on wireless tab then on the left side u will see Access Points tab, open this tab ..then click on Global Configuration tab. Now U can see the login credentials: please change here and then try it.
Be aware that AP have access via telnet.(If not then go to wireless>AP >ALL AP> click on AP name then go to Advanced Tab> enable telnet).