There isn't much you can do. Some internal hardware has failed. If unplugging it for a while doesn't help, try a factory reset. If that doesn't work and it's still under warranty, contact Sharp. If the TV supports a wired connection, you can use that if you can connect a cable to your router. You can always continue to use the TV and connect another streaming device to it (Roku or otherwise) if you can't resolve the Wi-Fi issue.
How to factory reset your Roku® streaming device | Official Roku Support
Where to get support for your Roku TV™ | Official Roku Support
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