Hello Miho1028,
Thanks for posting in the Apple Support Communities. We're happy to look into this with you. It sounds as though the process you're using is a forced restart. Given this, it would be expected that the device attempt to start back up on its own.
Were you following the guidance provided in article, If your iPhone won't turn on or is frozen - Apple Support? If so, did you complete all the suggested actions provided here? If not, you may want to continue with the article and perform the actions below under section: If your iPhone turns on but gets stuck during start up.
If your iPhone turns on but gets stuck during start up
If you see the Apple logo or a red or blue screen during startup, try these steps:
1. Connect your iPhone to a computer. On a Mac with macOS Catalina, open the Finder. On a Mac with macOS Mojave or earlier, or on a PC, open iTunes. Find out which macOS your Mac is using.
2. Select your iPhone on your computer.
3. While your iPhone is connected, follow these steps:
- On your iPhone 8 or later, including iPhone SE (2nd and 3rd generation): Press and quickly release the volume up button. Press and quickly release the volume down button. Then press and hold the side button.

- On your iPhone 7 or iPhone 7 Plus: Press and hold the side button and volume down button at the same time.

- On your iPhone 6s or earlier, including iPhone SE (1st generation): Press and hold both the Home button and the side button or the top button at the same time.
4. Keep holding the buttons until you see the recovery mode screen.

5. When you get the option to restore or update, choose Update.

6. Your computer will download the software for your device. If it takes more than 15 minutes, your device will exit recovery mode. Repeat steps 2-4.
If you're unable to complete the process, you can check out service options for your iPhone by contacting Apple Support at Get Support or by visiting iPhone Repair & Service - Apple Support.
Best regards.