If you’re missing calendars shared with you on Google Calendar, it is likely either a setting has changed on your Google account or an update to iOS corrupted the account setup on your iDevice.
Start by configuring your shared calendars that are displayed on mobile devices by logging into the Google account on your iDevice’s browser by visiting http://m.google.com/sync to view the settings for your sync account.
If that is configured properly then the issue is likely local to your device. Simply go to Settings.App (the settings icon on your iDevice), locate and tap “Mail, Contacts, Calendars”. Find the Google account in question in your list, tap on its name and then scroll to the bottom of the page, tap delete (this will NOT delete data on the account, but will delete the data cached on your device. The next step is to go ahead and re-add the account to your device. Once completed, you should now be able to view the shared calendars through Calendar.App (the Calendar icon on your iDevice).
Please feel free to leave a comment if this helped or didn’t.