I have a Samsung Galaxy S3, purchased on July 2012.
Sometimes, when I press the power button, the screen keeps black. The button LEDs turn on, but nothing else happens, and I have to remove the battery, or hold the power button for 20 seconds to force a reboot. It has happened to me 4 times; the first 2 were quite distant in time, but the last 2 were both this week.
Also this week, the phone has started to reboot itself. That has happened three times, two of them while I was using it, and the last one tonight, with the phone being idle (no wifi, no data connections, nothing). It just reboots automatically and then everything works fine.
I've never flashed the device, it's not even rooted. It's running Android 4.1.2, as I've installed all the official updates via OTA (base I9300XXELLA, kernel 3.0.31-836582, compilation number JZO54K.I9300XXEMA2). And no, I did not install recently any app which may be causing my problems.
So far, I've retired the microSD card (32GB card, manufactured by Samsung), because I've read that this may be the cause of the reboots. I'll try keeping it without the card for a few days. Although I've been using the same card for 5 months, and the reboots didn't start until this week. Edit: OK, we can rule out the SD card as the source of the problems - the phone also reboots itself without the microSD.
There are a lot of posts from people with similar problems, and most of the answers say they should try a factory reset. However, in most of the cases that doesn't seem to solve the problem. So, I didn't try it... yet. Maybe I should, but I'm afraid that the problem will be still present, and it's a lot of trouble for nothing.
I've already contacted with Samsung customer service, but in the meantime, is there anything else I can do?
Can I at least diagnose somehow what's happening? I've read here about something called logcat, however they say the log is only kept on memory, it's not stored, so it would be of no help to determine what happened after a reboot or a black screen problem...
Edit: the phone rebooted itself again, and this time the microSD card was not inserted. That means one suspect ruled out, but I still have to find the culprit.
Edit: it seems that the problems were solved after doing a factory reset. At least, the phone hasn't rebooted itself for the last 5 days...