My guess would be that your Roku updated its software in the last two days. You can check from Settings->System->Software update (or System update if it hasn't updated to 11.0). Look at the "Last updated" date. It shouldn't have lost your credentials, but things don't always go smoothly when Roku pushes out updates.
One other possibility is some kind of power glitch. If you're using the TV's USB port to power the Express try using the supplied power adapter.
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