If your iPhone 8 is stuck on a boot loop and is not turning on after replacing the back glass, there may be a problem with the flex cables or other components inside the phone. It is possible that the heat from the heat gun damaged one or more of the flex cables, or that the back glass replacement process caused other issues with the phone.
To troubleshoot the issue, you can try the following steps:
# Check the flex cables for any visible damage or loose connections. If any of the flex cables are damaged or loose, try carefully reattaching them to the motherboard.
# Try restarting the phone in recovery mode. To do this, press and release the Volume Up button, then press and release the Volume Down button, then press and hold the Side button until the screen goes black and the Apple logo appears. Then, release the Side button and follow the on-screen instructions to enter recovery mode.
# If the phone is still not turning on or is stuck in a boot loop, try restoring the phone using iTunes. To do this, connect the phone to a computer using a USB