I have the Roku Express 4K Plus and I have had it connected to my television before with no problem but now the TV says no signal. I unplugged and plugged back in the Roku device and on the screen it says Roku and it's like loading but then it goes the TV screen goes back to saying no signal obviously I'm getting power and obviously I'm getting a signal to the television because you can see the Roku screen come on. It's almost like it takes too long and the television thinks there's no signal in times out. Also I have the Internet connected and that is all okay. I don't understand what's happened.
If you're using the TV's USB port for power try using the supplied power adapter or another one from a phone or tablet. Some TVs cannot supply enough power via their USB ports for reliable operation.