iPad has been on charge all day and will not power on

If your iPad is unresponsive, connect your iPad to its Power Adapter and allow the iPad to charge. Then, while still connected to external power, try a forced-restart:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

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. Or if your iPad isn’t working correctly after you restart it, see the iPad Support website.

If you continue to experience difficulties, you will need to seek assistance directly from from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad:

Genius Bar Reservation and Apple Support Options - Apple

If your iPad has not been charged up for a very long period of time, it may take somewhat longer than 24 hours.

I had just neglected to keep my old 2010 iPad charged up ( it was dead and not sure how long I had left it for it to fully drain down to 0%, I usually keep all of my older iPad models, that I still use from time to time, always charged up ), it took about 30 hours for the battery to, finally, come alive and took another 6 hours, or so to fully charge up to 100% ).

So, you may have to give that iPad some more time to for the battery to come back from being fully and deep discharged.

Best of Luck to You!