Watch how I fix this problem in 1 min in 3 simple steps

Step 1: Search for calendar invitation you have received before

Step 2: Open the invitation, click on any of the link "Yes", "Maybe", "No", or "More options"

Step 3: Click on "Back to Calendar" link at the top left corner

There you go, the Google Calendar is back :)

 Let me know if it works for you too! Drop me some comments :)