I had this issue too, it resulted in a factory reset. The issue which you are experiencing might be due to the following reasons:
1. Software version is not up to date.
2. Temporary issue with the device.
3. Physical/liquid damages present on the device.
4. Software/Hardware issue with the device.
5. Third party application might trigger this issue.
To isolate the issue I request you to please perform the below mentioned troubleshooting steps.
Before proceeding with the troubleshooting steps, please check with the device Software, Currently this device operates on the Android version 5.1.1, Lollipop with a baseband version of G925TUVU2COI1. You can check the device software in Apps>>Settings>>About Device>>Software updates>>Check for update.. You can also check the baseband version in Apps>>Settings>>About Device>>Baseband version.
If your device software is up to date and you still experiencing the issue, I request you to perform the given below:
Step-1. Please perform the “battery simulation” on your device. It will refresh the device internal circuit. To perform battery simulation, Press and hold volume down key and power key for 30 seconds and check with the result.
If the issue persist, follow the next troubleshooting step.
Step-2. Perform the clear cache and clear data to “Contacts” application. Clear cache and clear data will delete temporary files present on the device. Please do not worry it will not effects the contacts saved on your device. To perform clear cache and clear data , follow the below steps and check with the result.
a) From Home screen tap on Applications/Apps tab.
b) Tap on Settings >> More/Application.
c) Tap on Application manager.
d) Swipe the screen from right to left and select "All" option.
e) Scroll down and select “Contacts” application from the list.
f) Tap on Clear data and Clear cache.
Please check the issue. If the issue persists, please follow the next troubleshooting procedure.
Step-3. Check the issue in Safe Mode:
Performing safe mode will temporarily block downloaded/third party applications. To enter the Safe mode, please follow the steps below.
Note: You can exit Safe Mode at any time by restarting the device.
1. Power the device off
2. Press and hold the Power key.
3. When the device vibrates; release the Power key and immediately hold down the Volume Down key
4. Keep pressing the Volume down key until the device boots up completely.
If successful, Safe Mode will appear on the device screen in the bottom left corner.
Please check the issue in Safe Mode. If the issue is resolved in Safe Mode, some third party/downloaded applications have caused the issue. We have to delete the application installed before the issue started.