how to fix ipad not start up?

Connect your iPad to its Power Adapter - and allow the iPad to charge for at least an hour. Then, whilst still connected to the power adapter:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

If iPad still doesn’t turn on, or if it gets stuck during start up, see the Apple Support article If your iPhone, iPad, or iPod touch won’t turn on or is frozen.

If restart remains unsuccessful, you’ll need to seek assistance directly from Apple Support - or the technicians at the Genius Bar of your local Apple Store.

Do you mean it literally will not power on and boot up? If so, that sounds like a hardware issue. For that, you’ll need to contact Apple support to see about hardware repairs. Start at -> https://support.apple.com/ipad