I am assuming this is due to a Sticky key. [Ref: Bootloop issue - Zorin lite]
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Boot up your machine
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When the letter starts printing press n hold
ctrl+alt+del -
It will take you to grub menu, Select "Start Zorin"
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If it boots up then you can confirm that it due to a sticky key. In this case you have to find out which key is causing the issue to fix it.
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If it still doesn't boot then let us know.