What do I do if my phone screen goes black and won't turn on I cannot reset as my lock button already doesn't work

Turn your device off and then force it to start. To do this, press Sleep/Wake and Home buttons and hold them for at least 10 seconds.

If this does not help, take a look these Apple Support articles:

If your iPhone, iPad, or iPod touch doesn't respond or doesn't turn on - Apple Support

Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support.

As a final step, try the recovery mode. Turn your device off, then plug it to your computer with the home button pressed. Keep the Home button pressed until you see the iTunes logo on the screen of your iPhone. After that on your computer you should see the iTunes window saying your iPhone needs to be restored to factory settings. Click on the restore button.

More info here: https://support.apple.com/en-us/HT201263

If these steps don't work, contact Apple and ask for help.

https://www.apple.com/support/iphone/contact/

Eimeart wrote:

Phome screen is black, Siri is still speaking but I'm unable to turn it lock button is broken

If the power button is broken, you're not going to be able to do much in the way of standard troubleshooting. Make an appointment at the Genius Bar at your local Apple Store.