iPhone 4 suddenly off,and then I can't open

Reset your iPhone

One of the first things you should do if you’re having a problem with your iPhone is to reset it.

You can do this by pressing and holding both the Sleep/Wake and Home buttons for at least 10 seconds. Don’t release the buttons until you see the Apple logo.

Update to latest iOS Software Update

If you haven’t done so already, update your iPhone to the latest iOS software update. We’ve heard that in some cases upgrading to the latest software update has fixed the issue.

Connect your iPhone to a Charger

If a hard reset does not turn your iPhone back on, then connect your iPhone to a charger. If your iPhone turns back on (with charge left), then it highly likely that your iPhone’s battery is defective. You can try restoring your iPhone to see if it helps, but if the problem crops up again, then you should take your iPhone to an Apple Store or Apple Authorized service center to get the battery or iPhone replaced.

Restore your iPhone

As the last resort, if the hard reset or upgrading to the latest iOS software update doesn’t work then restore your iPhone with iTunes.

Connect your device to your computer and then launch 3uTools, and check what mode your device is in as the following picture. And you could manually enter dfu mode or recovery mode.

How to Put your iDevices in DFU mode, Recovery Mode and Normal Mode?

Then you can restore it on 3uTools, if there is no hardware issue on your device, you can successfully flash your iPhone.