Audacity Mixer Toolbar Control Problem

[sevenor]

I've used Audacity many times before on my other/older computers with no problem but lately when I tried to capture the audio from a video clip (which I had done many times before successfully) I was prevented from doing because the Mixer Toolbar Control had been greyed out and therefore not clickable. I've tried fiddling with the Edit > Preferences and the Control Panel > Sounds And Audio Devices with no success. I'm not sure if it has any thing to do with my computer(maybe it's faulty for example)or its settings. Please advise. Thanks!