Screen is black on my iPhone 5S and holding home and power buttons has not helped. What else can I try?

My iPhone 5s does not turn on. I've attempted reset holding the home and power buttons but that has not worked. When connected to iTunes I receive the message "contacting the iPhone update server" constantly. When I try to try to STOP I receive message that the phone is detected in recovery mode and must be restored. What should I do next?