Magento doesn’t keep a log of changes unfortunately, they do have some functions you can use to add your own logs fairly easily, but not much is done by default (and this may not help you at this point) I haven’t used the last few releases, but don’t see any mention of this changing in the forums there. Depending on how many changes we’re talking about, you may be able to piece together what happened with server logs. MySQL has a query logging feature that may be enabled, but honestly, making the changes again may be easier and I don’t believe the log is on by default. Good luck with it.