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In other words, your Nexus 7 produces the Android logo / figure, but nothing else happens with it.
Obviously the Android OS is corrupted... and/or, some major device function without which Android OS cannot boot up, is defective damaged inoperative or missing.
Have you tried a hard reset? which wipes all user data off the tablet and reinitializes Android OS to its factory-issued state.
If you have tried it, did it go through or work, at all?
How old is your Nexus 7 tablet?
Has it ever been dropped or otherwise subjected to mechanical shocks?
Has it ever been gotten wet, or been turned On while conditions were at about 90 % humidity or more? has it been powered On at temperatures above about 88 degrees F or below freezing point?