You can use a RokuTV without a Roku account, but it just acts as a "dumb" TV. You're probably right - a guest linked it to his Roku account and then probably factory reset it when he left. I'm not sure how to get it back to "dumb TV" mode, but you should probably create a Roku account (they're free) and then investigate "Guest mode":
If you need to factory reset it again:
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