TV box won't start up - displays apple logo for a few seconds, flashes then repeats

Try disconnecting HDMI and power for 10 seconds and reconnect. If that doesn't work do a restore (see below)

• If you have an Apple TV HD and a Mac with macOS Catalina or later: Disconnect the power and HDMI cables from Apple TV. Connect one end of a USB-C cable (sold separately) to Apple TV and the other end to your computer. Open the Finder on your computer, select Apple TV in the sidebar, then select Restore. If that doesn’t work, contact Apple TV Support.
• If you have an Apple TV HD and a PC with iTunes or a Mac with macOS Mojave or earlier:Disconnect the power and HDMI cables from Apple TV. Connect one end of a USB-C cable (sold separately) to Apple TV and the other end to your computer. Open iTunes on your computer, select Apple TV in the Source list, then select Restore. If that doesn’t work, contact Apple TV Support.