Try plugging in the iPhone to the charger or to your computer and just leave it there for a while (45 minutes or so). I don't know why, but I have seen this resurect a few iPhones in the past.
One other thing, if you have a new or different Apple data cable laying around, use that one instead of the one you are currently using. You never know, cable could have failed.
That worked for me. It wasn't immediate but it did work!. My phone had turned into a black slab of nothing, was fully charged and stopped working while I was flipping through email. I had connected the "slab" to the computer and nada so after trying your suggestion and having a bit of patience, it is now working again. Why would my wonderful device do this to me?
Was late to work because my alarm didn't go off on my iPhone this morning! (4 w/ IOS 5) When I woke up, I went to check my phone and it wouldn't turn on; so I plugged it in to the charger and no response. I left it plugged in while I was getting ready...came back a shortwhile later...still no signs of life. At that point, I held both buttons for several seconds and the apple logo appeared and the lock screen displayed shortly thereafter. And what's odd, is that it showed that I had about 80% of battery life left. So it seems like something else caused it to shutdown completely. One note though...I updated to IOS 5 on Sunday...and I've used my phone ever since with no problems. So this is the first time this has happened since the update.
In looking at a thread on another site, I found this excerpt from an alleged beta tester back in June...and it was in reference to the iPad 2 with IOS 5, but seemed plausible enough to share here:
Issue: "Hey guys, whenever my iPad 2 is locked for a while, it wont wake from sleep. I have to give it a hard reset. Is anyone else having this issue?"
Response: "I think I isolated the issue....make sure banner notifications are turned off for mail app. If the screen is locked and you get a mail it freezes up causing you to reboot. hope it gets fixed in the next beta!"
Follow-up: "Did it help you at all? I tried recreating every way I could think of with mail and never could. I'll turn of mail notifications for the rest of the night and see what happens."
"Hey Rick, yes it helped, my iPad no longer shuts off. I turned off mail banner notifications like Defender2010 said, and at least for now, it doesn't turn off anymore."
Mine has just died too. I had it plugged into the mains yesterday so it was charing. It was about 90% done and the phone rang so I answered it. After a few minutes the call cut off and the phone just died on me and now I can't get it to do anything.
I have been on the forums for the last 24 hours and all the sugestions haven't helped. Doing a hard reset or trying to get it into DFU mode has enabled me to try and restore the device but when it tries to verify with the Apple Servers it crashes out with the same errors everyone was getting when we all tried to download the iOS5 update the other week.
I've tried it on 2 macs but nothing.
It's odd because as soon as I disconnect from the mains or laptop it just dies again.
Please can someone help