Amazon ec2 Ubuntu no ssh after update and reboot

I have Ubuntu 14.04 server (LAMP), running on Amazon ec2. On my client machine I run Putty with imported SSH key. Session is saved, so there is no changes always. I had no errors during connection and everything was fine. But today I've made apt-get update and reboot. Now I seeUsing username "ubuntu".Server refused our keyerror and promt to enter password. I've already tried to Stop and Start instace, but that doesn't help.