Re: Why has my lumia 550 stopped working
Try a soft reboot (hold power and volume down until your phone vibrates) for a simple restart. That's the first go-to step for any problem.
Failing that, try and see if calling or texting the phone results in any vibrations that might indicate that the phone is alive but the screen is an issue.
Ensure the phone has some charge in it, leave it for 10-15min plugged in.
That's about all I can suggest.