The subject of soap stars' absences is a frequent question on the AB.
Basically, it works like this. Cast members have an annual contract with a guarenteed number of appearences. During their 'down time', they are written out for several weeks or months, depending on storylines, or the actor can request time off for other work - theatre etc., subject to their contracts. For example, they can't radically change their appearence - years ago they were not even able to get sun-tanned on holiday, althougfh this rule seems to have been relaxed lately.
So, as is the way with soaps, people can 'vanish' for weeks or months, and they simply don't get mentioned, although listen out for occasional mentions of her name which will lead up to her return.
File that conundrum away with the well-worn list - why does no-one ever support Manchester United - why isn;t the Rovers' gents in Ken Barlow's living room - etc (contd. page 94).