I tried emailing this to their support first, and it didn't help, then I couldn't reply to their useless email either because of some other stupid reasons. And I have scoured Google and even tried sugarbombed but that hasn't helped yet.
Go to the xbox store - find fallout 4 and there should be an option to reinstall the season pass for free because you've already bought it - if not you should of received your billing invoice from Microsoft confirming your purchase on the day you bought the season pass.
Good Luck
Cheers
If you have redeemed it, you can redownload it for free. If that option isn't there, are you sure you redeemed it? Because even if the price changes, bought is bought and you can always redownload.
I never had to deal with Microsoft support.
http://support.xbox.com/en-US/contact-us
[edit]
I think this link might be more useful for you.
http://support.xbox.com/en-US/contact-us#4:1:1