If it is 10+ years old, then it's likely it simply died. It could be the power supply, but not sure what type it is. Unless you have a supply compatible with it, just consider replacing it. Also, do you have both on the same TV, or were you just testing the Roku 3 from another TV?
To be honest, just consider getting a new one, even if you have to get an HDMI converter. If you do get a new one, get the Express 4K or higher. Do NOT get the basic Express. You will be amazed at how much faster it is, and the Express 4K and higher have more than enough memory to hold all the channels you need without having to constantly re-download.