Micfrosoft Outlook Calendar Problem

I hope someone can help a tech dinosaur!

For many years I have used Outlook to bring together my main Aol.com email account plus a small number of auxilliary Aol.Co.UK and Gmail email accounts for ease of display of all of them in one place.

Recently I set up my first ever Outlook.com email account and added it to Outlook.

The calendar feature was previously tied to my main email account and I found it ideal for an electronic diary. For some reason the calendar has switched to the Outlook.com email account and all of my diary entries have disappeared.

How can I get back to old position of a tie to my main email account - I would hate to lose my diary entries!