my calendar has disapeared

I did something and now my calendar doesn’t appear - can you help?
Also, I would like to open the calendar in a different window from mail - how can I do this? (It was trying this that I seem to have lost my calendar)

With version 7.x if you right-click on one of the other sections like Mail or Contacts and make sure that Calendar is ticked.

In the upcoming version 8, you can open sections like the Calendar in different Windows. If you would like to give it a try, you can get the beta from the Release History