Please note, Audacity 2.0.5 and 2.0.6 do not officially support Yosemite.
Make sure in Force Quit that Audacity isn’t running.
Open Finder, Go > Go to Folder and type:
~/Library/Application Support/audacity/
Open audacity.cfg in TextEdit. Select all the text and delete it. Then type the following at the top of the file:
NewPrefsInitialized=1
Save audacity.cfg and restart Audacity.
If that does not work then perhaps some plugin you added is causing a problem. If so, make sure Audacity isn’t running, then edit audacity.cfg so it shows this:
NewPrefsInitialized=1
[AudioUnits]
Enable=0
GUI=0
[VST]
Enable=0
GUI=0
Rescan=0
Gale