Oh, that's not good.
The Typhoon, if it is too far away may lose the signal. Mine has done it many times.... When this happens it will turn around to the right and face home, then fly back. However, if you lose the video signal and touch the left joystick to go up (as you said), or right to move in any direction, it will respond because it's still receiving input from the ST16 although you see nothing on the screen. What then happens is that you think it's not responding but in fact you're still flying it.... into a tree, into a hill, into a lake. Not saying this is what happened in your case, but it does happen.
The Telementry logs on the ST16 will let you know what happened and if you were still connected. Yuneec will ask you to send them the FLIGHT LOG folder. If all is good, they may replace your drone free of charge.