how to stop flashing apple logo?

Try things in these links:

If your iOS or iPadOS device restarts or displays the Apple logo or a spinning gear unexpectedly, click here --> https://support.apple.com/HT203899

If your iPhone, iPad, or iPod touch won't turn on or is frozen - https://support.apple.com/HT201412

Apple logo with progress bar after updating or restoring iPhone, iPad, or iPod touch - https://support.apple.com/HT203435

If they do not work then follow the advice at the end of the documents about contacting Apple.