I understand that the alert tone for your timer doesn't end as it should. I can certainly understand why this would be a concern, and I'm glad to help!
Before you proceed, please be sure that you have a current backup of your data:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Since the issue persists after restarting your device, I recommend that you reinstall iOS as a next step. Please follow the steps below to reinstall iOS on your iPad:
- Connect your device to a computer and open iTunes. If you don't have a computer, try to borrow one, or go to an Apple Store or Apple Authorized Service Provider for help.
- While your device is connected, force it to restart.
- On an iPhone X, iPhone 8, or iPhone 8 Plus: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the recovery-mode screen.
- On an iPhone 7 or iPhone 7 Plus: Press and hold the Side and Volume Down buttons at the same time. Keep holding them until you see the recovery-mode screen.
- On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Home and the Top (or Side) buttons at the same time. Keep holding them until you see the recovery-mode screen.
- Don't release the buttons when you see the Apple logo. Keep holding until you see the recovery mode screen.
- When you get the option to restore or update, choose Update. iTunes will try to reinstall iOS without erasing your data.
iTunes will download the software for your device. If it takes more than 15 minutes, your device will exit recovery mode and you'll need to repeat steps 2 and 3.
If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support
Keep in mind that this article was written for a different issue, but these steps will allow you to reinstall iOS on your device.