Is there a light showing on your device? How are you powering the device? Is it plugged into the TV USB port, or is it plugged into the wall outlet with the provided cables? In addition, have you tried using another HDMI cable as well as trying your device on another TV to see if you are seeing the same issue with no signal?
Please let us know and we'll continue assisting you from there!
If you are still unable to resolve the issue, please send me a PM (private message) with your Roku account email address, and include the serial number of the Roku device from Settings>System>About along with a summary of the issue you are experiencing. I'll be able to assist you further from there.
Good luck! I've had similar problems and support is non existent. I spent 7 months emailing them and finally they agreed to replace my device only to not follow through. They said they would send a replacement device within 1 day of receipt. That was almost 2 weeks ago. They don't even respond to emails with anything more than a runaround.
I have the 2020 Roku Ultra and when I plug it in to the wall outlet the box is lit up showing me that I have powered on the device, but when I plug in the HDMI Cord to the TV from the Roku My TV keeps saying No Signal. I have switched TVs, and HDMI Cords and I still get the same thing every time.