Did you try deleting the .cpd file? That’s often a first step. Is there any source control at play here?
FYI, I tried deleting the .cpd; no luck.
Edit: posted this as soon as Jeff C posted his response. Hope my FYI wasn't interpreted as being catty!
I have a version of the build on SharePoint, which I guess I could bring down and see if the issue happens again. Whenever I work on these projects, I copy them down to my machine, make edits and publish, and re-upload to SP. But what's odd is this project was working fine. I've not published or uploaded or downloaded, it's just sat there on my C: and broken itself.