I have a Roku Ultra that I have never had a issue with. All of a sudden it goes out and says no signal. I have tried all of the HDMI on my TV and made sure that everything is plugged in well. I have seen The Roku welcome screen pop up a couple times only for a second just to go back to no signal again.
Basic question I know but...when you switched the HDMI cables, did you also use your TV remote (not roku) to select the correct INPUT/HDMI port in your MENU SETTINGS or INPUT SETTINGS? Some tvs automatically detect new device plugged in, others you have to point it to the device.
Last resort would be to do a hard reset on the Ultra. It erases everything as you would presume with no guarantee of fixing your problem. Look on the underside of the ultra near the front on the unit. There is a rubber button that says RESET. Press and Hold for a bit as they say and then see what happens.
If power is good, your HDMI cables are good, and RESET doesnt fix it, looks like replacement is needed through warranty or a trip to the store.
I believe it is a year. Let me see if I can find a link. I think you have to pay shiping and of course wait for replacement if they accept it. I have not had to use a warranty yet and mine are well past expiration anyways. i am sure it is less of a headache to just buy a new unit. Especially with corona.