In the opening period of the game, Boychuk was chasing the puck that was circling around the end boards when he was hit from behind by Canadiens forward Max Pacioretty, who was assessed a boarding minor. He remained down on the ice for a while as the medical staff in Montreal tended to him.
Eventually Boychuk's neck was immobilized before he was placed onto the stretcher with the help of a couple Bruins including captain Zdeno Chara and taken off the ice. The Bruins broadcast on NESN reported that Boychuk left the arena in an ambulance and was taken to a Montreal hospital.
Here's another look at the hit in GIF format courtesy of @Dr_Habs:
It's hard to completely tell but it looks like perhaps the worst and most unfortunate part of the collision with the boards is how Boychuk appears to awkwardly hit the dasher first. He appeared to be in a tremendous amount of pain as he lay in the ice.
Boychuk has averaged 21:05 minutes per game this season and in 26 games has a goal and five assists. He is one of their top penalty killers as well.