I've looked around this forum and others and can't find a solution to my problem.
I tried to restart my Samsung Galaxy S4 today and it came up with the: "Warning!! A custom OS can cause critical problems..." screen. I have tried pressing the volume keys as the message describes, and also tried holding down Volume Up/Down, the power button and the home button, which people have suggested would work. This just turns the screen blank, until I let go and the phone vibrates and returns itself to the Warning screen.
I have never tried rooting my phone and it had been working fine until today. Anyone got any suggestions or should I get in touch with my insurance/ network/ Samsung?