If it won't close, just restart your computer, and it should be okay. If you're having trouble opening it due to a source that's not loading properly (we're still looking in to a case where that can happen), you can reset your scenes/sources by these steps:
1. After restarting your computer, go to finder
2. Press Shift-Command-G
3. Paste in: ~/Library/Application Support/obs-studio/basic
4. Delete scenes.json
5. Start OBS again and your scenes will be reset to defaults.