CiviVolunteer Stopped Working with upgrade to 4.7.29
I'm not sure this was the problem, but the default PHP interpreter was at 7.1. I noticed CiviEvent was also not working (error message about "pass by reference"). I set PHP back to 5.6 and both CiviEvent and CiviVolunteer seem to be working.

The CiviCRM status screen is giving notice "PHP Out-of-Date: This system uses PHP version 5.6.32. This meets the minimum recommendations and you do not need to upgrade immediately, but the preferred version is 7.0."

Update July 3 2018: Currently at CiviCRM 5.2.2 with CiviVolunteer 4.7.31-2.3.1 and was able to move to PHP 7.1.18 with no issues.