Bootcamp Windows doesn't start up after 10.7 fresh install upgrade to 10.10
Found the problem. So it seems that it's a relatively common problem with Yosemite messing up the partition scheme, deleting the header for the Windows partition.

This guy called Loner T had been helping people on the forum, and one of the victims of the bug had written up a summary of the steps to take to fix the issue. Do note however, that the fix is highly technical, and requires at least a competent understanding of operating systems and the CLI to safely apply it. So if you're uncomfortable (or have no idea what the instructions are asking for) it's better to find a professional to help). Obviously it goes without saying that you should have a backup of all your data before you attempt this.

Link to the fix:
https://discussions.apple.com/message/27109000#27109000