Hi
There is no reason I can fathom why your Google Calendar hasn't returned. As soon as you log into your Google Account on your phone, contacts, calendar etc all return. Google also backs up automatically. I would check to see that your Google Calendar online is still ok. If it is the one on your phone should be just the same. If you have more than one Google Account make sure you are signing in correctly. I know that's simple but is often missed. When you start up your phone after the reboot make sure you are clicking all the relevant 'sync with Google' etc .
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