Trouble after upgrading regular advanced cable to HD box

Hi, hoping someone has experienced this or has any advice for me. I recently learned I could get an HD advanced TV box for no additional a charge. I took my regular box to the cox solutions store and swapped it out for the HD one. When I hooked it up, it went through to the TV correctly but would not load properly. So I took it back for another box. This time, it won't even go through to the TV via HDMI. It says no signal no matter what input I put it in.

I've tried three HDMI cables, and checked everything and it won't go through to the TV at all. How else can I troubleshoot this? Thnx