Hi @gwynlee ,
Try starting the phone in safe mode and see if it starts OK.
If it does:
Go to Settings > Apps > Scroll to find and select the game app that you think caused the problem > Storage > Clear cache > OK also Clear Data > OK. Then go back to the home screen and restart the phone in the normal manner and see if it works OK.
If it doesn’t
Try wiping the cache partition:
1. Switch off the phone
2. Press and hold the Volume Up key and the Power key simultaneously
3. Release the buttons when the Android recovery screen appears on the display.
4. Use the Volume Down key to scroll and highlight Wipe Cache Partition.
5. Press the Power key to select the entry.
6. Confirm the operation again by selecting Yes. Use the Volume Quiet key to do this. Now the Wipe Cache Partition will be executed, which usually takes only one second. Then select Reboot System Now.