Iphone doesnt start and flashing apple logo

If the forced restart does not help

1. Press and quickly release the volume up button.
2. Press and quickly release the volume down button.
3. Press and hold the side button.
4. When the Apple logo appears, release the side button.

You can try to put your iPhone in recovery when connecting it to a computer

1. Make sure that your Mac is up to date. If you're using a PC, make sure you have the latest version of the Apple Devices app or iTunes.
2. Connect your device to your computer with a USB cable.
3. Open the Finder on your Mac, or open the Apple Devices app on your PC. If your PC doesn't have the Apple Devices app, or your Mac is using macOS Mojave or earlier, open iTunes instead.
4. Keep your device connected and wait until the Connect to computer screen appears.

• iPhone 8 or later, including iPhone SE (2nd generation and later): 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 Connect to computer screen. 

1. Locate your device on your computer. When you see the option to Update or Restore, choose Update. Wait while your computer downloads the software for your device. If the download takes more than 15 minutes and your device exits the Connect to computer screen, let the download finish, then repeat step 3. 
2. After the Update or Restore completes, set up your device.

Get more help

• If you see an error when you update or restore your iPhone or iPod, learn what to do.
• If you can’t update or restore your device with recovery mode, you can't use recovery mode because of broken buttons, or your device is stuck in Activation Lock, contact Apple Support.