I see that you're having an issue when trying to search for events in your calendar after updating to iOS 11. I use my calendar multiple times a day, so I understand your concern, and I'd be happy to help.
Let's start by having you restart the iPhone and then test to see if the issue remains:
Restart your iPhone, iPad, or iPod touch
If after the restart the issue still remains, then let's have you review the following article, which includes signing out of and back into iCloud and see if the issue remains. To sign out go to Settings > [your name]. Scroll down and tap Sign Out. Enter your Apple ID password and tap Turn Off.
Get help using iCloud Contacts, Calendars, or Reminders
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