My grandson gave me his old Samsung Galaxy Note 4 a few moths back and I think I have wrecked it. Was working great until Saturday when I took photos then used bluetooth to transfer them on to the computer. About 1/2 hour later it switched itself off and no way would it start again. I took the battery out a few times and still nothing. In the end I left it for a few hours then tried again, and it fired up.

Was all good till half way through the next day when it switched off again. Eventually found on the internet about pressing volume, home and power keys. I got it to light up with the pic of a green alien (looks like it). Lots of tiny print in the top left screen and part of it said "could not do normal boot, mmc_read failed". Downloading do not turn off target. I don't have data on the phone so if it was trying to download, no idea how it could. Anyway that screen lasted ages then the phone switched itself off. Nothing seems to get it started again.

Any clues what on earth I have done to this?