You might be seeing a well known bug, if it applies address it here https://support.apple.com/en-us/HT208332
Do you know what version of iOS is on your phone now? Was it up to date?
You might be seeing a well known bug, if it applies address it here https://support.apple.com/en-us/HT208332
Do you know what version of iOS is on your phone now? Was it up to date?
Try https://support.apple.com/en-us/HT203899
If nothing in there works for you, after making a backup:
1) Make sure iTunes is updated on your computer
2) Plug in your phone and then hold down the home button and sleep/wake button until you see the iTunes logo.
3) When iTunes pops up a dialog box asking you what to do, select "update" and let it do its thing.
If that doesn't work you will need to contact Apple Support directly.
Software: iOS 11.2.5. Yes up to date.
Already tried that!! Didn’t fix it.
iPhone 6s Plus turns off randomly and locks up on black screen after phone calls