Some celebs have chosen to withdraw in circumstances where a bye was not possible or desirable e.g. Jimmy Tarbuck's ill health, Kelly Brook's bereavement, John Sergeant deciding to leave the show.
Other celebs have been told they are fit to dance despite their injury e.g. Laila Rouass, Patrick Robinson.
Three celebs have received a bye in Strictly history - Tina O'Brien, Jade Johnson and Natalie Gumede. Tina returned after a week and stayed until elimination, Jade had to withdraw because she was unable to return after one week, and we don't know if Natalie will be able to come back yet.
Each couple is entitled to one bye throughout the whole series, provided the doctor recommends that the celeb doesn't dance that week. So if they don't return within a week, or they can't dance later in the competition for whatever reason, then they forfeit their place in the show.