returning to WOW after many months

I haven't played for many months but have kept my account paid for. When I tried to play today I was unable to log in because it seemed to be trying to install an update. When I allowed this it looks like it is installing the whole game again. Did I do something wrong or is this what I have to do because I haven't played in such a long time? Thanks.

It's because you haven't

It's because you haven't played in so long.