If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
Try to survive and thats all you can do.
WASD/Cursor keys – movement
Mouse – look around
Left mouse button – fire (you can shoot only when you are outside of trench)
R – reload
C/Left Ctrl/RightCtrl – crouch (crouch to evade explosion shards)
Esc – exit