iPhone black screen of death

So your iPhone is stuck with a white Apple logo stuck on a black background. You've searched the Internet and somehow ended up here...

I've found that if you try the following you can get it restart again ok:

1) Put the device into restore mode 

2) Hold down the power button till it reboots

3) Once it starts rebooting try clicking the power button and/or home button for a few seconds at a time

4) It either restarts normally, or try again - go back to step 1/2

I know it seems a bit random but it can work if nothing else does. Though it probably indicates there's a hardware problem with your device if this keeps happening even after you do a full restore or two...

Update (Oct22): If you're seeing a black screen on a newer iPhone (e.g. 10+ onwards) with only three buttons (i.e. power, volume up and down) then you can try the hard reset sequence:

1) Tap on volume up

2) Tap on volume down

3) Hold down on the power button until it reboots

UKC19TRACING - What is it?

If you've been taken here after scanning a UK COVID venue QR code - your device offering to search for "UKC19TRACING" - then to make use of this QR code you'll need to load the UK NHS COVID19 app on your device so you can "check in" to the visited venue where the QR code is displayed. This is the app that is based on the purportedly privacy preserving Apple and Google Exposure Notification system so in theory your 'check-in' is anonymous and probably preferable to writing down your address/phone at said venue.

On an Apple iPhone/iPad when you scan these QR codes from the lock screen camera it will attempt to search for "UKC19TRACING" - which why you may have arrived here. Curiously this doesn't happen when the device is not locked and instead says "No usable Data found". 

If you extract the content from one of these [big!] QR codes (with one of the many QR code scanning apps) it starts with "UKC19TRACING" and is then followed ":1:" and after that the main section is encoded in JSON Web Signature (JWS) [See RFC7515] format. Basically this consists of the following; after the "UKC19TRACING:1:" part there's three sequences of base64 separated by a dot("."), the second of which contains the venue name and postcode, and the other contain the signature and associated details - see here for more details.

If you're interested in creating a QR code for your venue see the the relevant gov.uk site.