Dear addenic,
I am really sorry to hear this, but I might have a few fixes that you could try...
1. Shut Down your Mac cmpletely
2. Wait at least 15 seconds
3. Turn on the computer
4. Try another cable to connect your device with (just in case)
5. Make sure that you go to Quick Time Player > File > New Movie Recording > Next to the Red button the little arrow facing down (press that), Make sure it records your device and it does have a tick next to the device you want to record. Next go to Sound and make sure the sound is also ticked somewhere because i had past experiences with no ticks anywhere.
If all this fails you should drop QTP and use AirServer to AirPlay the screen to your computer and go to Quick Time Player > File > New screen recording and record the whole screen...
Hope this works,
Adam