Every few months my Roku will freeze, reboot and get stuck at the startup screen (problem started sometime in the past year). I discovered that taking the sd card out will allow it to boot to the home screen again, if the sd card is put back in without formatting it won't mount, if it's formatted before putting back in it works fine. Mentioning it now because it's happened twice in the past two weeks (was happening once every few months) and I saw another post mention the same thing happening to their Roku 3 recently.
Roku 3 4200x 8.0.0/4143-04 2gb sd card
edit - with all my regular channels installed the sd card shows 7% used