Q: I want to reorder an iPhoto calendar but the buy calendar tab isn't live.  What can I do?

What is your iPhoto version? If it is iPhoto '08, version 7.1.2  or earlier, you can no longer order Print products from Apple. In that case you need to upgrade to iPhoto 9.6. See:  Apple Photo Services: Product line details for books, cards, and calendars

If your iPhoto version is supported, are you still in a country, that supports Print products? Check the "Print Products Store" setting in the iPhoto Preferences, Advanced panel. Is it set to your country, and dies this match the country of your AppleID?

If none of the above applies - is the "Buy Calendar" button active, if you create a new calendar, just for testing? You can cancel, after pressing the Buy button.

This is just to test, if there is a problem with your iPhoto Library or theparticulare calendar.

What has changed during the last four days? What is your iPhoto version? Have there been any iPhoto crashes? Have you updated or upgraded software?  Moved your iPhoto Library?

Make another test, to see if your current iPhoto Library may be causing the problem. Create a new iPhoto Library in your Pictures folder, import a few photos, and create a calendar in this library. Does the "Buy" button work in this library?

I guess it must be something to do with OS X yosemite that I have upgraded to and now perhaps iPhoto isn't compatible.

We still don't know your iPhoto version, what is it? Only iPhoto 9.6 is compatible with Yosemite. If you have an earlier version, there may be compatibility issues. Only, if you upgrade to iPhoto 9.6 from a much older version, the layout of the calendars and books might change, because each iLife version had different templates.

Try the following:

1 - delete the iPhoto preference file, com.apple.iPhoto.plist, that resides in your

     User/Home/Library/ Preferences folder.

2 - delete the contents the following folder: User/Library/Containers/com.apple.iPhoto

3 - reboot, launch iPhoto and try the Buy button again.

NOTE: For Mavericks and Yosemite,  go to your Home folder and use the View ➙ Show View Options menu to bring the this window:

where you can check the Show Library Folder checkbox.