There's game-breaking bug which may happen when you kill Revenant boss. In my case after killing, there's nothing happens. Judging from letsplays there should be scripted attack by Bony Whoops (those annoying flying drones) from the headquarters of Heskell Institute. When they attack, at the same time they break glass windows that separated arena of Boss fight from headquarters and pave the way into for the player. But nothing happened. I guess script broke. In this case I used "noclip" console command and it helped. You can activate console command by pressing "Shift" and "~" at the same time. then you type "noclip" and press "Enter" button. After using "noclip" and if you don't need it anymore you can again activate console and type "noclip" again (and then press "Enter" again). It's cheat and I don't like to use cheat, but I guess there's no choice.

Judging from absence of such topic before I guess that it's very rare bug in the game. I don't know what was the cause, but it happened and soured the impression.