My iPhone XS Max won't turn on. I've tried all the techniques and nothing works. What should I do?

If you did this and it still did not work, get an appointment at an Apple Store. https://support.apple.com/en-gb/HT201412

As you have tried all of the techniques you will never be able to use the phone again. Unless you missed this one:

• Charge the phone for 30 minutes
• Press and quickly release Volume Up
• Press and quickly release Volume Down
• Hold the button on the right side of the phone until an Apple logo appears (15-30 seconds).