Method 1: Install the latest updates on the device
Simply installing the latest updates on your Lumia 640 can improve its stability and also introduce patches designed to get rid of the random reboot bug to the device. To check for updates on your Lumia 640: Go to Settings. Tap on Phone Update. Tap on Check for Updates and allow the device to check for the latest updates and inform you of any that exist. Alternatively: Download and install the Lumia Software Recovery Tool. Open the Recovery Tool. Connect your Lumia 640 to the computer. Wait for the Recovery Tool to recognize your Lumia 640. If there are any new updates available for your Lumia 640, the Recovery Tool will inform you of them and ask you for permission to download them.
Method 2: Use the Stop Restart App
For Windows Phones that did receive updates that fixed the random reboot bug or could not be updated, Microsoft created the Stop Restart App and published it on the Store. The Stop Restart App, once installed, examines a Windows Phone, determines whether or not it is affected by the random reboot bug and whether or not it needs to be fixed and then tries to fix it. Installing and using the Stop Restart App is another method that you can use to fix a Lumia 640 that keeps rebooting randomly.
Method 3: Perform a Soft Reset
Performing a Soft Reset is another excellent candidate for the method that is capable of fixing a Lumia 640 that keeps rebooting randomly. To perform a Soft Reset, simply press and hold your Lumia 640’s Power and Volume Down buttons until the device reboots. Once the device boots up, it should no longer reboot at random times.
Method 4: Perform a Hard Reset
If nothing else works, your last resort is none other than a good ole’ Hard Reset. Performing a Hard Reset has pretty decent chances of fixing your Lumia 640, but it will also wipe all of the data stored on it, so make sure that you back up all of the data that you want to keep before you proceed. Go to Settings. Tap on About. Tap on Reset your phone. Follow the onscreen instructions and confirm the action whenever you are prompted to do so. Once the Lumia 640 resets and reboots, set it up all over again and it should no longer reboot randomly.
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