I have the same problem… my power button ceased to function after owning the phone for 5 years. i have had an on/off issue with the phone not receiving cell signal, so I used the assistant menu to restart the phone. When it was off, it didn’t finish the reboot instead just shutting off. I’ve essentially lost all my contacts and half of my pictures being that VZ Cloud was changed to a subscription service and all of my data was deleted from the cloud. By taking the battery out and putting it back in, the screen lights up with the “Samsung Galaxy S4” screen then promptly goes black. I have repeated this many times and to no avail. Anyone got a solution? this sucks