So, if your outputting device gives you manual control over its display resolution, consider setting that to auto, or picking your TV's best resolution. This is a big problem for PCs, for example, since they're usually capable of outputting video in a wider variety of resolutions than an Apple TV or Roku would be. Most TVs are smart enough to adapt to any resolution within their capabilities, but some devices that send video to a TV can be annoying in that regard. If neither of those solves the problem, one final, common thing to try is to check resolution settings. It might seem silly, but it's a fairly common reason why the No HDMI Signal error might appear.
It's also worth checking if the source device is connected to power. If it's not working, it might be an issue with the device's HDMI port or the HDMI cable.
If another device is successfully connected to your TV over HDMI, unplug it and plug the new device into the same port. Plug the device into the different HDMI ports on the TV and try each one. If this doesn't work, the next step is making sure the HDMI port is working properly.