If it is only the one songfile, there is a chance that file is corrupted.

If it should happen again on a New file, suspect the need to perform a Return to Factory Settings under the Options window. Band in a Box writes to a volatile file upon closing, which updates user settings and a host of other things, any time BB does not get the chance to exit gracefully can corrupt that file. Return to Factory Settings restores a known working version of the file and can correct a host of issues.

--Mac