Hi Laura,
I had the exact same problem with my 40" Sharp Roku TV. I powered it up, expecting the Home screen, but instead it would show the start-up screen "SHARP - Roku TV" for a few seconds, go black for a few seconds, and then show the start-up screen again. In an endless loop.
It turns out that there is a factory reset button on the lower back of the set. It is located close to the 3 sockets for RCA composite video and sound (small holes color-coded yellow, white, and red).
Mine is a tiny button that can only be pressed by a pen or paper-clip ending.
I simply powered up the set, used an ink pen tip to press and hold the tiny button for 20 seconds, then released it.
What followed was the TV asking to be set up exactly as when I first bought and installed the set, as in wi-fi network to connect to, password, e-mail address, etc.
All theme settings were gone, but all of the channels I had originally selected were still there, albeit in random sequence.
Here is the Roku Support webpage and section that mentions this very thing:
https://support.roku.com/article/208757008#section-3
Hope this helps.