Hi @Raymond28
Rather than referring you to BNB let me explain what I think happened.
Your guest rather than going into the booking and altering his dates, notified BNB that YOU as the host wanted to cancel the booking.
BNB would then have sent you notification that the guest said you wanted to cancel the reservation. Because it is worded in a confusing way, you probably accepted it.
Hence it showing as a cancelled booking.
Call BNB now (contact details 1st post in the Community Help forum) and explain what has happened they will see your messaging between yourself and your guest and hopefully amend the booking and lift the cancellation.