My nexus 7 (2013) can not start up after Android updated

A new Android version (?) was updated to the tablet while it was running well with Youtube. Suddenly, the screen turned black, then Android restarted to the end of Optimizing Apps (34 apps), and the screen turned black again to repeat this cycle endlessly.

I shut the tablet down by pressing and holding the Power button, then powered it up. The cycle repeated.

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