What to do if the screen of iPad or iPhone is frozen & black screen
If the screen of your smartphone is frozen, only a forced restart can help. The explained procedure does not delete your data, but simply forces a restart. This also works if your screen is completely black or the screen is frozen, i.e. it no longer responds to any touches.
iPad, iPod Touch (up to the 6th generation) or iPhone with home button: Press and hold the side button (left picture) or the upper button (right picture) together with the home button for at least 10 seconds. If successful, you will see the Apple logo.
iPod Touch (7th generation) and iPhone 7, iPhone 7 Plus: Press and hold the side button and the silent button on the opposite side for at least 10 seconds until the Apple logo appears.
iPhone 8 or later models: Press and release the Volume button (1), then the Volume button (2), then press and hold the Side button (3) until the Apple logo appears.
Device does not start despite forced restart
Connect your iPhone or iPad to a power source with the original Lightning/USB cable and AC adapter connected. The charging screen should appear after one hour at the latest. If you don’t get any information about the charging status of your device, check the charging connector. Also clean it, as dirt can hinder the charging process. If you have another cable available, try it out as well to avoid possible damage.
You can also connect the device to your PC and try an iTunes backup. If the device still remains black after a few hours, there may be a defect on the screen and you should contact Apple support.