Lost Windows 10 Activation

[Windows 10]

Hello,Yesterday the message "Activate Windows" appeared on the bottom right of my screen. I bought a legitimate copy of Windows 10 about 2 years ago and I haven't made any hardware changes. I tried to go to settings -> Activation -> troubleshoot, but it says it can't find a product key. I went into my Microsoft account purchase history and found the Windows key I bought. When I click on it, the URL says "Not available". How can I fix this?

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