Being relatively innocent and lacking really good knowledge about SSH keys exchange order, I set up a new instance on Amazon EC2. Downloaded the key pair PEM file, connected and immediately started to cook my dream test server. Although , the Unconscious Lazy Stupid Dork in me awakened for a moment and after installation of libssh2 I forgot to add my key to ~/.ssh/authorized_keys file. Of course, the evident consequence of this lazy intolerably stupid behaviour was complete locking out of the newly created and manually sharpened instance I just spent 2 hours upon. With heavy heart I deleted it and started from scratch one more time.
Dude, if you do not understand how important is this, believe me, it is better to know in advance and follow this simple instruction than rip off your remaining hair after being excluded from all the fun by your own stupidity.