Hi, there. Not sure if this will help you, but I found Galaxy S4 trapped in a “Warning a custom OS can cause critical problems in phone and installed applications…” It gave me the option to push volume down to cancel and restart, but the volume down button did nothing. Removed battery, nothing. What worked? First, believe it or not, I prayed to ask God to help me with this issue. Then, after removing the battery, I restarted the phone with just the Power Button and Home Button pressed at the same time. It worked! So, my recommendation is to pray and try the Power Button and Home Button pressed together to start the phone up. Good luck!