First of all, please check your user code and password combination using https://useraccount.metu.edu.tr/. You can also specify recovery e-mail address from that page. If user code and password combination is not the case, VPN Client profile could have been changed. Therefore, you are advised to remove the VPN connection profile from your device. You can clear saved profiles from your VPN Client or you can re-install the VPN Client program. If your problem persists, please visit http://faq.cc.metu.edu.tr/faq/i-have-problems-vpn-services-what-can-i-do