Lollipop 5.1 upgrade, now I can't open my tablet. What should I do?

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AC Question

Lollipop 5.1 upgrade

I upgraded my Nexus 7 yesterday evening successfully to 5.1 Lollipop.
I switched on this morning but the tablet will not go past the starting up screen. I have switched off once or twice by continually holding the start button but it will not open the tablet successfully.
What is the answer?