From Top > Troubleshooting and Error Messages > Archives disappearing on Blogger:

Question

Why are my Archives disappearing?

Answer

What is means: some configuration changes on our end have resulted in some occasional errors for some people's blogs, where one week or month's archive disappears from their archive index. Nothing is ever deleted or lost in Blogger (your posts are safe in our database), so the problem is simply that the archive index isn't properly being built.

How to fix the problem: the first thing you want to try is resetting your archives. Go to your archive settings, and change your archiving frequency to ,No Archive. Save your changes and publish. Then go back to your archive settings again, and select the monthly or weekly option you had previously. Save your changes, then go back to your archive list and hit the republish all button.

Better remember that. I was showing Bill something in my blog that I'm going to bump up to a Saturday post only to find out half of my archives were missing.