There have been many reports of charging issues occurring with iPad9 subsequent to updating iPadOS. With iPadOS 26.6 installed, you'll need to update to the current version of iPadOS - at the time of writing, this being iPadOS 26.7.2 - or optionally upgrade to iPadOS 27.2.
But first, you'll need to recover the iPad to a working state. As is, the battery is likely fully depleted. Connect your iPad to its Power Adapter - and allow the iPad to charge, undisturbed, for at least 24-36 hours. For this issue, if you have an older 12W USB Type-A Power Adapter, this may actually restore charging more quickly than using a USB-C Power Adapter that uses USB PD (Power Delivery).
Many reports indicate that the iPad will very slowly charge - and then recover normal operation. When the iPad does restart, keep it connected to its Power Adapter while you perform the system software update.
If you remain unable to power-on the iPad after 48hrs, you would be well advised to seek assistance directly 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 from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:
https://apps.apple.com/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 hands-on test and assess your iPad. An AASP may be much closer to you than your nearest Apple Store:
Genius Bar Reservation and Apple Support Options - Apple
In addition, for those affected by this specific issue, it would be in your best interest to submit a Bug Report via Apple's Product Feedback portal - as until Apple receives a sufficient volume of reports, it may not assign resources to investigate and resolve the issue. For iPad and iPadOS:
Feedback - iPad - Apple
When submitting a report, ensure that you include details of your iPad model and the installed version of iPadOS within the body pf the report. Don't rely on the pick-list alone as this may not include the most recent version of iPadOS.
You are unlikely to receive a personal reply, however, it is our understanding that all feedback is logged, analysed and forwarded to the appropriate team(s).