Why does my phone freeze when turned on then restarts over and over

my phone fell and the flex cord stopped working but I got it fixed, then it fell in sea water n my back camera hasn't worked since, soon after the phone froze n went off, everytime it comes on it freezes n goes back off, I've tried flashing, resetting, it works sometimes when I give it n older mini version like 12.5.9 but that's after I've played in it n keep putting it on, but anytime it goes off it goes back restarting and sticking again.