Hi -
Apparently this is an occasional issue in some versions of GB (are you using GB IOS or macOS?). One possible solution reported is to duplicate the track (Cmd-D) and Copy & Paste the problem region(s) to the new track—apparently this sometimes restores sound.
If that doesn't work, a few other troubleshooting steps to try:
• When you click on the Drummer region, does it turn gray? If so, that would indicate that it was somehow muted—normally the "M" key would toggle that
• Check that no other track is soloed
• If there are other regions on the track, check to see if they play, or if all regions on that track have no sound
• Close the Project without Saving and re-open it, and see if the Drummer sound comes back
• Save, and then close the Project and re-open it, and see if the Drummer sound comes back
• Do a hard computer shutdown/restart—that sometimes fixes odd one-off problems
• Try Drummer tracks in other GB Projects, to see if the issue is repeatable, or specific to only one Project