You'll need to contact Microsoft about the issue. In order to link the account, you had to authenticate to it, and it's the same to unlink it. You can try a different browser to see if cookies or storage cache is messing with things, but if that doesn't resolve the issue, then something might be going on. Between when you first linked it and now, something would have changed with your account authentication, and that'd be something that Microsoft need to work with you to resolve, so definitely contact them to try to figure out what's going on and a way to fix it.