It's possible that one of the binary files in the project got corrupted and rebuilding it fixed the issue.
If you're still running the unpatched 2.8 version of CodeWarrior you might want to download the 2.8.1 patch from the Metrowerks website. You can check what version you have by going to Help/About Metrowerks CodeWarrior and clicking the Installed Products button. If you're already running 2.8.1 or greater then no patches are required.