While typing this topic, I managed to find the issue. Just putting this out there in case anyone is hitting the same issue.

ResultsViewer stopped working for me. When trying to start the program, nothing happens, no error is produced.

To fix the issue:

• Just try opening the ResultsViewer a few dozen times. Randomly, the program will occasionally start.
• When open, clear all available settings under 'Preferences'

Other steps I've tried, which may have contributed to the solution:

• logging on/off & rebooting
• reinstalling openstudio (actually upgrading from 1.5.0 to 1.6.0)

Last time it working normally, I was working on an .sql file produced by a custom .idf file, not one produced by Openstudio.