After the crash, Chuck was near death. Blair made a "pact with God," promising that if Chuck lived, she would follow through with marrying Louis and never be with Chuck again.

Blair has been hiding away, planning her wedding with Louis while secretly spending time with . While Chuck and Louis suspect an affair, Dan is actually her only confidant regarding her religious vow. Serena’s Cover-Up The End of the Affair? | Gossip Girl Wiki | Fandom

Season 5, Episode 11 of Gossip Girl , titled , is a pivotal turning point that deals with the aftermath of the tragic car accident involving Chuck and Blair. Episode Overview

The episode confirms that Blair lost her baby as a result of the accident.