Ouch. Very sorry to hear this happened, and that you didn't know what to do to rescue it. There are 3 things in fact that work almost every time to save that situation - the 2 methods SC mentioned, and Red Button short presses would have done it as well, each one dropping it 1 ft until it is in hand-catch range... the 4th method (holding throttle to minimum for at least 20 seconds while the craft inches its way downwards) also works sometimes, but not every time. There is a 5th method too, which I mention last because it has only worked for me once, which is to rise to 100 ft, leave it there for 10 seconds, and try the descent again.
Yuneec UK told me that my report of this behaviour was the first they had ever heard of it - I'll be interested to hear what they say to you.
I have my own theory about why this happens occasionally, if you are interested, which you could help corroborate or disprove by looking at your flight logs. I think it has to do with massively inaccurate barometer readings and the effect that has on the TH's normal staged descent.
If my theory is correct, your barometer-driven altitude reading should be way off your actual height, and more importantly, at the time of descent, way off the height offset that was established when you took off , which are the circumstances I have noticed every time this has happened to me. If you'd like to see this in action I made the video below which demonstrates it. Watch my hilariously inaccurate altimeter readings, which begin correctly pre-motors, but rapidly go weird immediately after motor start and then throughout the launch, only stabilizing (at a wildly wrong value) about 30 seconds later...
In this case we have the first refusal to descend at
08:41 (throttle continues to be held low, but vertical speed reduces to 0) which lasts until
09:15, when method 4 above works for me and I can get her back down just by relentlessly holding throttle at minimum...